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.




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.

A Few Interesting DIY Audio Projects c. 1955

Circuit for a simple bass/treble parametric equalizer that uses no inductors.  Could be  a useful piece.  Not sure if 250k/500k dual pots are still available?  Since I have still not finished building the last tube EQ project that I described, so I’m not likely to craft this thing anytime soon.  If anyone out there picks this up and builds one, LMK…

Going back to AUDIO magazine in the early 50s: a few circuits that caught my eye. As always, if any of y’all out there build any of these, drop us a line and let us know how it went.

A ‘cascode’ preamp which features very high gain and defeatable compensation for phono cartridge.  I am not aware of any available mic preamp that uses this circuit.  a lot of gain available here…

A low-gain preamp that mixes two high-impedance signals to a medium-impedance output.  Add a few transformers (500: 50K inputs, 15k:600 output) and this could be a useful tool for selecting/blending two mic signals to one track of (tape).  IE put a couple of mics on a gtr amp, blend to taste… I generally do this using submasters (busses) on the console but here’s a potential way to do it that uses a much simpler signal path, IE right from the mics into the convertor (or tape machine).

A 50-watt ultralinear power amp using 6550 power tubes

A 12-watt 6V6 power amp with some negative feedback and a pentode input stage.

A simple 8-watt power amp that uses a 6SH7 input tube and a single 6L6 output.  This circuit uses a great deal of negative feedback in order to supposedly get more linear response with cheap output transformers.  I’ve never used the 6SH7 tube before…  curious though.

A simple design for a variable sawtooth wave generator.

A DIY tube mixing console; plans, schematics, methods

Some more interesting bits from AUDIO magazine c 1955:  plans and schematics for a radio station mixing console.   Very little in terms of equalization devices, and no compressors discussed, but a wealth of ideas for low-level and line-line level amplifiers and mixing circuits.  Download the entire piece as a 4-page PDF:

DOWNLOAD: DIY_Console_1955

There is little of real interest here in terms of a complete system, as the system discussed is a mono mixer; but if you are interested in some simple, novel mic preamp and line driver plans, you might find this worthwhile.  Personally I am drawn to the OD-3 regulator tubes; i know zero about voltage regulation tubes; I have always used Zener diodes in the past when i needed regulated voltage; but the very simple way that these tubes are used here leads me to believe that they are pretty easy to work with.. Might wanna try them out.  They are just $5.15 at AES.

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.


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.

Hi-Fi in Mexico 1955

A two page article from AUDIO magazine circa 1955.  Consider this article in comparison to “Japan Goes Hi Fi.”  How differently these two countries have developed since these pieces were written.   Both certainly became manufacturing powerhouses, but Mexico has been much less able to keep the profits from those operations in their country.  I have owned plenty of audio-objects made in Mexico (Fender guitar amps come to mind) but I have personally never seen a Mexican hi-fi brand.  Anyone?    Also: please note: this article is presented for historical reference only; I do not endorse the political/ethnic generalizations and characterizations that author makes.

Hi Fi Amplifier Schematics c. 1954

The Craftsmen C-500 power amplifier

Today we have some hi-fi amplifiers from the mid 1950s to look at.  Many of them use unusual output transformers with split primaries, so I am really not sure if it’s even possible to build these designs today.  The Craftsmen C-500 pictured above uses a standard PP-VC output transformer; it also uses readily available, robust 6SN7 and Kt66 (aka 6L6) tubes.  Check out the circuit in the output cathode bias; even though this is a cathode-biased amp, there is a provision for adjusting the overall bias, as well as a balance adjustment ( to allow the use of unmatched power tubes).  Here’s a few more…

Bogen DB15 schematic

General Electric A1-300 power amp.  This is a good simple design that will provide 12 watts.

The Leak TL-10 amplifier

The Quad II pre-amp and power amp.

A DIY hi fi preamp design from 1954.  Note the very sturdy parallel-cathode-follower output (the 12AU7 section).  This could probably drive a 10K load pretty easily.

1955: Japan Goes Hi-Fi

Download a three-page 1955 article entitled “Japan Goes Hi-Fi” as originally published in AUDIO magazine.

DOWNLOAD: Japan_Goes_hifi_1955

The article concerns an early Hi-Fi show in Tokyo.  Postwar Japan had sufficiently moved beyond the subsistence level to indulge in luxury-leisure pursuits; soon Japan would come to dominate the world in the electronics field.

The ‘hi-fi coffee shop’ pictured above may seem like a quirky anomaly, but you would be amazed at the sound systems on display in even modest Japanese bars and restaurants.  The last time I was in Japan, we stayed at a bed and breakfast in the mountains of Hokkaido; upon arrival at the inn, my jaw dropped when I saw that the sound system in the lounge consisted of a JBL Paragon speaker and huge McIntosh tube amplifiers.  It’s hard to say exactly why this trend developed, but if you spend any time in Japan, I think it’s clear that ‘excellence’ in general is an important concept in Japanese culture.

Above is a 1955 advert for The Panasonic.  Not ‘a Pansonic,’ but THE Panasonic.  The Panasonic was a single product introduced for the US market by the Matsushita corporation: an 8″ full-range hi-fi driver.  Panasonic would soon grow into an entire global electronics brand, second only to SONY as an ambassador of the Japanese electronics industry.

Here is a 1955 review of “The Panasonic:”

Previous Japanese audio culture coverage here and here.

Pro Audio hardware of the early 1950s

The General Electric (GE) BA-5-A Limiter

Continuing our review of the first two years of AUDIO magazine, today we will look at some of the more interesting bits of pro audio kit in evidence during 1954/1955.  AUDIO magazine had just made the transition to its new moniker in the wake of the introduction of the AES Journal (Audio Engineering Society), and for the moment, AUDIO sill covered a bit of the pro audio equipment that would soon largely leave its pages.

The GE BA-5 pictured above is, AFAIK, the largest and most complicated analog audio compressor ever made.  Although it has much less tubes, it’s kinda even more sophisticated than the Fairchild 660/670.    Here’s the schematic if you are interested.  From what i recall,  the BA-5 works by creating an ultra high frequency sidechain to obtain the control voltage value for the compression; I can’t recall the details at the moment but the basic concept was to allow the unit to have huge amounts of compression with very fast timings, but without any pumping or dipping artifacts.  Which was also the intent of the 660/670 design.  If anyone out there has a better explanation of this monster, please chime in.

The General Electric BA-6-B remote amplifer/mixer

The General Electric BA-9-A compressor, a much more basic pro audio compressor.  Circuit is essentially the same as the Gates Sta-Level.  The BA-9 is also known as the the uni-level; schematics are readily available online.

The General Electric BA-1-F plug-in preamp and BA-12-C plug-in power amp

The Hycor 4201 equalizer.  Similar to a Pultec program EQ but without the makeup gain amp; the Hycor is a fully passive device.

Langevin 5116 modular preamp

The full Langevin modular line of 1954: 5116 preamp, 5117 power amp, and 5206, 5208 power supplies

Another remote amp from 1954 – the Magnasync G-924.  Looks very cool.  Magnasync would soon be merged with the Moviola corporation and become a brand name for sound-for-film equipment.  See this previous post for an experiment with the Magnsync URS device.

An early ad (1955) for the Altec 604 duplex loudspeaker.  The 604 would remain a studio-standard recording/mixing monitor speaker well into the 1970s.

Okay this is getting pretty tech-y but here’s an advert announcing some new-ish tubes you might want to consider: the Tung-sol 12AX7 and the 5881 (AKA ruggedized 6L6).   Transistors were on the market at this point (1955) but were a ways off from reaching the performance and reliability that these great tubes offered.

Studio Microphones of 1955

The Altec 639, 633, 670A, 660, 671A, 632C, M-20, and M-11 microphones

From the pages of AUDIO magazine in 1954/55: new and new-ish studio microphones at the dawn of the transistor age.  Check em out…

The Shure 530.  This was Shure’s mid-range ‘pencil’ mic in the 1950s.  At the top end was the 525; lower in the line was the 535.  See this link for some audio tests including the 535.

Reslo and Bang & Olufsen ribbon microphones were marketed and distributed in the USA under the ‘Fentone’ banner in the 1950s.  Fentone continued as a brand-name for microphone marketing into the 1960s, but later Fentone product was not of this quality.


Above is the two-page advertising spread that introduced the Electrovoice 666 to the world.  The 666 is the grandfather of the much-beloved Electrovoice Re20, which has been a favorite of recording and live-sound engineers for decades.  Like the Sennhesier 421, the RE-20 is somewhat proof that dynamic microphone technology hasn’t really advanced much in the past three decades.





Above, a 1955 advert for the 666 along with a review of the 666 from AUDIO magazine.

Above is a 1955 ad for the Electrovoice 664.  The 664 was the ‘public address’ aka ‘cheaper’ version of the 666; the 664 was immensely popular and thousands remain today.  If you come across one of these, there is a good chance it will still work fine.  In terms of visual recognition,  I would argue the 664 has an iconic appeal exceeded only by the Shure 55 series and the RCA 77 and 44 series.  The 664 is not a terrible sounding microphone, although i don’t recommend actually using it in a modern high-decibel PA system.

Wrapping up, here’s an ad for all the microphones that Electrovoice considered its ‘professional quality’ in 1955: the 666, lesser-variant 665, 655, its lesser variant 654, 646 lavalier, 650 and 535.    Hear my pair of ancient 655s in-action here…