Tag: RCA

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

Post author By chris
Post date 06/04/2011
9 Comments on 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.

Tags custom audio equipment fabrication, gold coast recorders, listening tests, mic preamps, RCA

RCA Technical

Building an (almost) RCA OP6 Mic Preamp

Post author By chris
Post date 05/31/2011
26 Comments on 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.

Tags custom audio equipment fabrication, mic preamps, RCA, schematics

Concert Sound RCA

RCA Mixing setup at the Hollywood Bowl arena c.1955

Post author By chris
Post date 05/18/2011
8 Comments on RCA Mixing setup at the Hollywood Bowl arena c.1955

You gotta wonder where this thing ended up. Consider the history that passed through these faders.

Tags mixers, public address system history, RCA

Pro Audio Archive

Audio Engineering Magazine Pt 6: audio equipment

Post author By chris
Post date 05/09/2011
No Comments on Audio Engineering Magazine Pt 6: audio equipment

The earliest advertisement for the Magnecorder tape machine that I have seen. BTW get ready for a large collection of original Magnecord, INC documents from one of the company founders: on this site: soon.

In the sixth and final installment of this survey of pro audio of the 1940s, as related through the pages of Audio Engineering magazine, we’ll look at some of the more interesting pieces of equipment on offer.

1948: Altec announces the 604 duplex loudspeaker. The 604 would become a staple in recording studios well into the 1970s.

…and Ampex announces the tape recorder that would change the world. in 1948 it offered performance that would not be outdated until the 1980s.

The Arlington EA-2 ‘Audio Compensator,’ an early active EQ. Well, maybe ‘active’ is the wrong word, but unlike other equalizers of the era, it functioned without insertion loss. I would love to see the schematic for this unit if anyone has one to send over…

The Presto 41A limiter and 89A amplifier. These units were intended for master-disc cutting.

RCA jacks, panels, stands, and racks of the late 1940s.

OK this scan really sucks but I was excited to find an ad for my beloved BA-2 preamp. My advice: buy a UTC A25 on eBay and build a BA-2. Simple and excellent.

Tags altec, ampex, arlington electric, magnecord, presto, RCA, tape machines

Technical

Audio Engineering Magazine Pt 4: Schematics

Post author By chris
Post date 05/07/2011
5 Comments on Audio Engineering Magazine Pt 4: Schematics

Today we’ll look at some of the more interesting audio-circuit plans and schematics from the first two years of Audio Engineering magazine. Pictured above is a great lil’ amplifer (approx. 30watts) that uses a single 6AS7G tube for push-pull output. I’ve never used these tubes, but they are real cheap. Apparently they are sorta like 2 2A3 triodes in one envelope. Except that they cost $13, rather than $200 for a vintage 2A3.

A fully-balanced 30-watt amp using about a million dollars in tubes.

another 30-watt amp, this time using the very cheap 807 tubes. The 807 is similar to a 6L6, except that the grid connection is on a top cap. 807s can also handle crazy high voltage.a

A schematic for the venerable Langevin 108C, which was apparently a very popular choice for industrial audio distribution in the ’40s.

Another 807-based amp; this one is an RCA Radiotron model 515. This looks like a great circuit. Wish I had kept those 807s i found last year…

Tags langevin, RCA, schematics, vacuum tube hi-fi

Microphones

American Microphones of the 1940s: Audio Engineering Magazine Pt 3.

Post author By chris
Post date 05/06/2011
5 Comments on American Microphones of the 1940s: Audio Engineering Magazine Pt 3.

The Amperite Velocity Microphone

Continuing our review of Audio Engineering Magazine, today we will look at some of new offerings in microphones available to audio engineers in the late 1940s.

The Electrovoice 635