Tag: custom audio equipment fabrication

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Altec Custom Fabrication

Altec 438A – Modified for modern music production use

As we discussed in previous posts, the Altec 438A is an audio compressor designed in the late 1950s primarily for use in public-address sound systems.  It has a microphone pre-amp built in, and an absolute minimum of controls:  one knob determines the level of the signal that hits the input of the compressor; turn this knob all the way down and you hear silence; turn it high enough and you get a highly-amplified version of the input signal; keep turning it up and you get a highly amplified signal with the peaks attenuated or ‘compressed.’  There is also a 2nd input with slightly less gain; this unbalanced 100k ohm nominal input shares the same volume pot as the mic preamp.  I suppose that the 438A was useful enough in its day, as there are hundreds still to be found; but for use in a modern music-production environment, it’s pretty useless.  The levels are all wrong, there is not enough control of the compression parameters, and there is no proper balanced line-level input.

Here is how I took an original-spec 438A and modify it so that one could use it alongside more modern compressors in a music-production studio.

(From Left to Right): A switch to determine if either the two-stage mic preamp OR a balanced 15K nominal signal hits the input transformer of the compression amp; a threshold/ratio combo control ala the later Altec 436C; (top) an balanced output level control/pad ala the Gates Sta-Level (see here for details); (bottom) a release time control ala the Altec 436C.  The original volume pot still functions as an interstage gain control for the mic preamp.

Here’s the unit with the face flipped down, revealing the wiring to the newly added pots and switches.  At my client’s request I used 11-position detented pots; ALPHA makes these in a huge range of values and Mouser stocks them for just about $2 a piece.  A great value IMO.  I used sliver-plated 24ga stranded wire for the audio wiring and 22ga solid copper for the control signal wiring.

This 438A is now ready for use in any situation where a gentle, vintage compression sound is desired.  The input and output levels are what you would expect from a standard pro-audio compressor; the release timing is widely variable (but never very fast – this would cause some artifacts due to the way these simple vari-mu compressors function); and the threshold/ratio control will yield a wide range of results as well.  Since the unit was in good physical and electrical shape, no re-tubing or re-capping was necessary.

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Custom Fabrication

Leslie Rotating-Speaker Interface Unit – UPDATED

The Leslie Model 51 is a single-speed, large Leslie unit with 2 amps, a 15″ woofer, a rotating horn, and two stationary side-speakers.  It sounds killer, and we wanted a way to get any audio signal off the board and into the unit.  Here’s how we did it for about $100.

The Leslie 51 was originally designed to by driven and controlled by the Leslie model C2C preamplifier.  Here’s the schematic for the C2C, taken from the original manual for the 51.

The critical things to take away from this:  the 51 has a 6-pin vacuum-tube type socket which supplies input power, ground, two unbalanced audio channels (600 ohm nominal) and a control pin to turn the leslie motors on and off.  Pin one is Ground, 2 is control, 3 and 4 are 117 V AC, and 5 and 6 are the two audio channels.  The motors are turned on simply by connecting pin 2 to pin 3.  In our simplified control unit, this motor-control function is accomplished with the simple footswitch you see below.

Balanced input jacks go into Jensen JT-11-FLCF series 600/600 transformers (AKA the cheap unshielded ones).  These are used just to unbalance the lines going in, allowing the lines leading up to the interface to remain fully balanced.

The only other parts:  an IEC receptacle, a fuse holder, power switch, and the 6-pin tube socket to mate with the Leslie cable.  Enclosure is a steel Hammond.  When I fired it up, it worked fine, but there was some hum and buzz when the motor was engaged.  I figured that this was likely due to the dried-out 30 foot Leslie cable with 3 117V AC lines running parallel to the 2 unbalanced audio lines.  Even at 600 ohms that will likely cause problems.  I built a new 10-foot cable using Canare starquad for the audio lines and voila.  I made the female cable end with a 6-pin tube socket, and the male cable end from the busted-off base of a… i think it was a 42 tube.  Anyway.  Problem solved.   As beautiful as a Leslie sounds with an organ, it’s absolutely genius with guitar.  This thing does require a pretty hot level to get it up to full volume, but it’s easily within reach of any piece of +4 balanced outboard gear.   Even using it with the humble Guitar Pod sounds fantastic.

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Custom Fabrication Gold Coast Recorders Technical

RCA OP-6/BA-2 Hybrid Mic Pre Amp: Listening Test/Shootout

Alright!  So earlier this week I described the successful completion of the RCA OP-6/BA-2 microphone preamp.  Check out this previous post for all the construction and technical details.   The short story is: the RCA OP-6 is one of the most fetishized vintage mic preamps out there; I have always wanted to try one out; the easiest/cheapest way for me to do this was to build one (or at least as close as I could get).  The problem is that the input stage requires a special attenuator device, exact values unknown; therefore I had to substitute an input stage from another device.  I chose the input stage from the RCA BA-2, as I have built many of these and they always work great.  The result: a hybrid of the OP-6 and the BA-2.

OK so there it is.  Anyway, the very helpful+generous TW came by to help me out on this one.  I wanted to try the OP-6/BA-2 Hybrid (hf. OBH) on a couple of different sources with a couple different types of mics. We a/b’d the OBH with an API 512.  I use the API 512 as a benchmark for mic-pre shootouts because it’s a high-quality unit that many people own and use regularly.  What you are about to hear are identical mics tracked through the two different preamps, direct to Pro Tools via a Lynx Aurora.  Levels were matched. No other processing, level adjustment, or manipulation was done.  You are hearing exactly what came out of the preamps.  To appreciate the differences between the units, you will need to listen to these files on good headphones or a full-range speaker system.   If you listen on a system with a subwoofer (we used the Blue Sky system at Gold Cost Recorders), you will hear some dramatic differences.

OK.  So first up: let’s listen to the drum kit above.  These are vintage ludwig drums, 30″ kick, 12″ and 14″ toms, 14×5 wood snare.  Cymbals are fairly dark old Zildjans.    You are hearing two identical Shure SM-81s placed right next to each other, approx 8 feet in front of the kit, pointed directly at the kit.  The 10db pads on the SM81s are engaged.  The SM81 is not the prettiest sounding mic, but they have a very flat frequency response.

First: here’s the API 512:

LISTEN: Drums_API

…and here’s the OBH:

LISTEN: Drums_RCA_hybrid

Our impressions were as follows: the OBH has more low end extension.  On the Blue Sky system, the kick drum in the OBH signal moved the room in a way that the API simply could not.  The API seemed to move the kit a little closer to the plane of the speakers, but at the same time the top end was not as in-focus.  There is a definite low-midrange boost going on with the API.  I can say this with relative confidence because I measured the frequency response of the OBH and it is totally flat from 15Hz – 10K, with only a very slight raise above 10K.  In terms of operation: the API gain control was at 3 o’clock; the OBH was at 9 o’clock.  WOW that is a lot of gain.

Next, let’s listen to some acoustic guitar.  TW played an old Martin D-19 (same as a D-18) that i mic’d with a well-matched old pair of Beyer M260s ribbon mics. The M260 has a built-in gentle roll off that starts at around 200hz

Alright so take a listen.  First, the API 512:

LISTEN: AcGtr_API

…and now the OBH:

LISTEN: AcGtr_RCA_Hybrid

Our impressions were that the OBH had more low bass but less low mids; it had a more ‘mellow’ feeling.  The OBH also had better high-end extension.   This also resulted in slightly more HVAC (air conditioning) room noise in the OBH.  Although I like the sound of the OBH again here, it is less of a clear-cut choice.  The mid-boost that API seems to deliver is very welcome in this particular setup.

In summary: TW put it this way: ‘(the OBH) is like a pair of gentle shelves (shelving EQs) on the very highs a lows.’  I think this is very accurate.  The OBH seems to give what I think of as an English sound: that sort of larger-than-life, hyper-real sound that UK records have always aspired to.  I highly encourage your DIY’ers out there to give this project a shot; you will find it to be a very useful tool.

Thanks again to TW for his help with this listening test; T’s band THE STEPKIDS is just back from LA where they did a direct-to-vinyl (!) live set in front of a studio audience (!!!) at Capsule Mastering Labs.  Check out the details of this very cool endeavor here and here.

 

 

 

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Custom Fabrication Technical

UPDATE: Vacuum Tube Program EQ project – Part 2

The vacuum-tube powered program EQ that I described in this previous post is nearly complete.  This thing has sat on the shelf for a while, as it was a little daunting dealing with all those parts inside a 17x4x3 chassis.  So far so good, though…  very excited to fire this thing up in a session.

Here you can see the frequency-selection switches all wired up.   The switches are 5-way, single-deck switches with the exception of the hi cut, which requires a 2-gang switch.  The EQ has separate boost and cut sections, like a pultec, with 5 selectable frequencies for each.  The amount of boost shelving is variable via a pot, while the attenuation controls are fixed cut.  It would be easy to make the cut controls variable as well, but as you can see I am a little short on space!  I suppose I should use a 3U enclosure the next time.  Alright stay tuned…

UPDATE: This piece was finished, and it sold to a prominent producer/engineer before I had the chance to record a thorough demo of it.  It worked out very well. The circuit performed as-advertised, and the only problem was that it had a little too much gain – enough to use a mic preamp in the majority of situations.  Since it has sold it’s been used on several major records; when these albums are released I will provide links to some of the sounds tracked through it. 

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RCA Technical

Building an (almost) RCA OP6 Mic Preamp

(image source)

The RCA OP-6 ‘Portable Amplifier’ is one of a handful of truly visually-iconic vintage mic preamps.   The OP-6 was designed as a “remote,” as-in, ‘on-location’ single-channel mic preamp for radio station broadcasts.  It uses three 1620 (6J7) tubes  – most mic pre designs of the period use two.  Furthermore, the 1620s are in pentode operation rather than triode.  The result: a ton of gain.  95db, apparently.  This is almost twice as much gain as the classic RCA BA1 and BA2 mic pres.  OP-6s are in high demand – click here for a seller asking $2900.   Further indication: Blackbird Rentals in Nashville has thirteen in stock as rental units.  The first time I heard an OP-6 in use was at Blackbird; I was producing/directing a live-in-the-studio performance for Martina McBride and John McBride was engineering; he was using the OP-6 for something… I can’t remember what exactly. Anyway, it caught my eye and we talked about it a bit; later I learned that he has an especially strong appreciation for these units.  A strong endorsement coming from the man who likely owns more vintage pro audio gear than anyone who ever has or ever will live.

The OP-6 schematic is pictured above (this file is readily available in high-res on the internet).  So anyhow, if we consider how in-demand the OP-6 is, it seems striking that no one offers a modern equivalent for sale, even on a small-scale level.  If you take a close look at the circuit, the reasons become apparent.  There are two big obstacles to re-creating even a semblance of an OP-6.  First is the input attenuator:

Sure, it’s a voltage divider; probably constant impedance; but what exactly are the values?  And what about that value of that feedback path issuing from the attenuator back to the input stage?  If I could get my hands on an original OP-6 and open up the attenuator, sure I could maybe sort it out.  But I imagine that re-creating that part on custom order could cost hundreds of dollars; frankly I have no idea.

The second obstacle to re-creating the OP-6 is the output stage choke.

The choke is designated L-1 in the schematic.  Curiously enough, it’s actually physically part of the output transformer.  Based on this fact, there is zero chance that this was an off-the-shelf choke, say a UTC for example, that we could track down.  OK – but the crucial value of a choke in a circuit like this is the inductance.  So long as the voltage an current handling values are sufficient, any choke of same inductance should give a similar result.  Now again, if I had an original OP-6, I could measure the inductance and maybe a current off-the-shelf part exists to satisfy the requirements.  But… I don’t think anyone out there is gonna send me their prized OP-6 to open up.  And $2900 is a pretty stiff R+D cost.  So what do we do?  Well, in the absence of any actual electrical engineering training, I looked for some good advice and then I guessed.

The very friendly+talented John Atwood sent me the diagram above; in response to what I can’t recall. This diagram explains why the choke is necessary in order to get the best possible performance from a single-ended vacuum tube line output stage.   Looks pretty similar to our OP-6 output stage, right?  Based on this… I made a wild guess.  I ordered the very inexpensive Hammond 156C choke.  150mh inductance with 8ma current capacity.  8ma might be a little low, but I have found that Hammond really undersells the specs of their transformers, so I’m not worried.

Alright so now we’ve got a choke that might work.  What about that input attenuator?  The best course of action would probably be to get a used 100k Daven T-pad, but without implementing the feedback path that the stock OP-6 attenuator has, it seems like this is slightly pointless vis-a-vis maintaining originality.  So instead: I used the input stage from my favorite, yes yet again, the RCA BA-2.

The BA-2 schematic is pictured above. The input stage uses a 1620 tube, wired as a triode, with a 100k ohm pot following it.  So I just took this input stage, up to the pot output, and wired it in front of the second two OP-6 stages (starting at the grid of stage #2).  When I did this, the whole system worked fine except when the volume pot was a zero (IE., when the grid of tube #2 is shorted to ground).  This caused weird noise and a little humming.  Not sure if this is due to the negative feedback in that stage, or if this is simply a general characteristic of 1620s when they are run in pentode – but it sounded awful.  The easy solution?  I added a 1M resistor from the grid to ground, and isolated this from the pot with a 1K grid-stopper resistor.  Done and done.  The BA2/OP6 Hybrid is born.

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The finished piece is shown above.  I used a 6X5 rectifier tube like the OP-6 uses; in fact, with the exception of using DC filament voltage, I kept the power supply the same as the OP-6.

The audio portion of the circuit is shown above.  What with the three pentode stages and feedback paths, this is extremely complicated for a mic preamp.   A lot of parts.  On the left you can see my usual Jensen 115 input transformer.  Audio caps are a mix of Solen, sprague, and some random Russian ones.  Basically whatever would fit.  Resistors are random as well; now that I have confirmed that the piece works well, I will probably replace the carbon-comp resistors in the B+ chain with some quieter modern resistors.  On the right is the output transformer.  I used an edcor 15K: 600; $10.22!

Even though this edcor is unshielded, there is zero hum following the volume pot.  And as for performance?  I did a frequency sweep through the entire unit; response is absolutely flat from 15hz to 10k hz.  At 10k there is a rise of about 1db up to 22khz, at which point response begins to fall of pretty rapidly.  This is really excellent performance considering the inexpensive transformers.  I did not measure the gain but there is a lot of.  The output level can get extremely hot.

Anyhow.  That’s it for now.  As soon as I have a minute I will provide some audio test examples; I’ll post some A/B examples of this unit versus an API 512, hopefully with both acoustic gtr and drums.

Thanks to DW at EMRR for helpful suggestions regarding this project.

***UPDATE*** Listening test has been done and results are posted here.

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Altec Custom Fabrication Technical

Altec 436 Compressor: Taming the output level: part 2

In a previous post, we looked at the Altec 436 vari-mu tube compressor.  I built one of these a few years ago, and it never really got used all that much because the output level is so hot.  The 436 is a very primitive compressor design, and it sounds awesome- but it was also built primarily for service is installed sound systems: industrial paging use, etc.  The stock 436 circuit adds a lot of level to your signal if you have the input level set high enough to actually cause significant compression.  I built an external attenuator box using a 600-ohm Daven T-pad attenuator and a UTC transformer to re-balance the signal, but this was not really an ideal solution.  The box was pretty big and heavy and I generally could not be bothered with taking it out and setting it up.

The solution came to me when I was examining the circuit of the Gates sta-level.   Here’s the schematic if you want to take a look.   Now, this may look a lot different than the Altec 436 schematic, but the differences are not too significant – other than the fact that the gates has a regulated power supply, the circuits function in the same way; the main difference is the particular types of tubes that are used.  Both are fully-balanced vari-mu compressors which are staged as (input transfo)-(attenuator pot)-(vari mu input amp)-(driver stage in Gates only)-(output amp)-(output signal rectified, timed,  and sent to grid of input stage to regulate input stage amplification)- (output transfo).

The Sta-level, however, has an output level control, whereas the Altc 436 does not.  So how do they implement this?

Pretty simply.  5 resistors and a normal linear taper pot give us an output loss that we can vary between 10 and 16db, while still maintaining a safe operating impedance.  In all fairness: the BEST way to do this would be to use a balanced H-pad variable attenuator, which would give us the ability to vary the output from NO loss to, say 20db or so; but balanced H-attenuators are crazy expensive and very large physically; too large to fit inside an altec 436 chassis, certainly.   Another option would be to use a variable T-pad after the output transformer, and then add an additional 600/600 transformer after the T-pad in order to re-balance the signal (there is a certain vintage vari-mu compressor that works this way, but i can’t seem to recall which; anyone?).  This solution is also not ideal from a cost and size perspective, although it would certainly be less expensive than the balanced H attenuator.

Anyhow, the major downsides to the ‘Gates-solution’ are: 10db loss is inevitable; output impedance will vary slightly with use of control; variation range is limited to 6 db span.  Well; i used my output modded 436 in a session yesterday, and for what its worth, here’s what I can tell you:  the minimum 10db loss is welcome – it put the 436 into the same basic operating range as my 1176 and Distressor; the impedance mismatch (into a Lynx Aurora) did not cause any audible problems that i could detect when used on guitars and drum machines; and the 6db control range was fine as well – i was dialing in levels and getting sounds into Pro Tools with no fuss.

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Before I drilled a fresh hole into the face of my 436 clone, I built the intended circuit into a tiny outboard box.  When the design confirmed itself, I added the circuit into the 436 itself.  But about this little test-circuit box:  it’s lightweight enough that it can simply hang off the patchbay, supported by the patch cables themselves.  Now every vintage mic preamp that I have can be given variable output control quickly and easily.  This will allow me to dial in extra-gritty sounds using the preamp gain control (which is generally interstage rather than input or output), and then use this little device to get the level back down to an appropriate level to hit the convertor.

If you own a vintage Altec 436 or 438 and you find that you have to battle the high output level, I highly suggest that you give this modification a shot.  It’s very easy to simply build it into an outboard project box at first in order to see how you like it before you drill a hole in yr Altec.  And if you use any vintage tube gear in the studio: try making one of the little boxed-versions of the circuit.  It will really open up some new creative and sonic possibilities for the gear you already have.

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Altec Technical

Altec 436 Compressor: Taming the Output Level

Ah the classic Altec 436C compressor (see here for previous coverage on PS dot com).

Here’s a fresh high-res scan of the original product-sheet (2pp):

DOWNLOAD:  Altec_436C

I built one of these some years ago and it really sounds great.  I used UTC ouncer -series transformers.  Don’t be fooled by the tiny size – these are very good units.  In fact, Ouncers are used in the early Urei 1176 as well as UA-175 and 176 compressors.

You can see how super-simple the circuit is. Aside from using a conventional power-supply circuit (rather than the voltage doubler that the original unit uses), i built mine pretty much exactly the same as the schematic.

Now, if you look at the schem, you can see that there is no provision for an output control.  This is a problem because these things add a lot (like 20 db or more) level to whatever you put into them if you have the input control high enough such that the unit is actually compressing.  In the past I have gotten around this by using an outboard Daven H-pad attenuator that I mounted in a little box.  This is not ideal for ergonomic reasons.  Anyhow…  while studying the (very similar) Gates Sta-Level schematic the other day, I was intrigued by the very simple, very inexpensive variable balanced output pad that the Sta-Level uses.

It’s like $5 of parts. Five 1/2 watt resistors and one pot.   Based on information in the Sta-Level manual, adding this circuit after the output transformer of the 436C will provide a minimum 10db and a maximum 16db attenuation.  Perfect.  Now, it’s true that using this control will vary the effective output impedance of the unit slightly; but according to Gates, “This pad has been carefully tested to assure that the small impedance mismatch resulting from this range adjust-ment will not affect frequency response or other characteristics. ”   And Gates was writing this back in the day when the Sat-Level would almost certainly be seeing a 600ohm load.  Considering that nowadays it will more likely see a bridging load of 1500 – 10k ohms, I think it’s safe so say that this circuit should be (at least as) sonically-transparent (as a vacuum-tube vari-mu limiter can be).

Gonna dig up some matched 160 ohm resistors and give it a shot…  more to come…

 

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Custom Fabrication Technical

Recent Custom-Build Stereo Amplifier (Home Use)

Above: a very minimal single-ended stereo amplifier with very low gain, designed to accept headphone-level input from an iPod or Laptop and drive a pair of loudspeakers.

The circuit design is extremely simple – Each 1/2 of a 6SN7 feeds the grid of a 6L6 through a .1uf capacitor.  There is a 3-stage power supply with a choke filter before the B+ hits the output transformer.   I have built a couple dozen of these, and the design works well – the sound is very clean and direct; the small 8w output transformers do roll off the very low end, but I have never found this to be an issue with the music that I listen to.   Easy to add a powered sub to the system if one was into rap or heavy orchestral music. BTW, I use this very same circuit, with the same components, for music listening at home; I have mine connected to the output of an Airport Express, powering a pair of Bose 201 speakers. For instances when customers have wanted to hook the unit up to a line-level (rather than headphone-level) output, I substitute a 6SL7 for the 6SN7.  This requires simply changing the cathode resistors on the input tube socket and gives 3x the voltage gain.

What interests me in this design is not the mundane circuit – it is the overall appearance/sculptural aspect of the unit.  I arrived at this particular form through consideration of the appearance of the vacuum tube; as much as possible, I have tried to make the overall complete unit an amplified echo of the tube itself.  The circuit is laid out extremely carefully and the components/wires color-coded (red for B+, orange for audio), green for grounds); it’s my perhaps naive hope that someone unfamiliar with audio circuits could look at/into this piece and maybe gain some understanding of the way that a tube audio amplifier works.