Tag Archives: diy

A Simple One-Tube Compressor Circuit c.1953

1954_DIY_limiterReader Bill W recently sent us a few interesting ancient DIY audio projects that we had not seen before.  Today’s post is from RADIO-ELECTRONIC ENGINEERING and was written by one Ed C. Miller.

DOWNLOAD THE COMPLETE ARTICLE:Constant Output Broadcast Amplifier

The article details the operation and construction of the above-illustrated broadcast-remote mixer/amp, which includes a limiter.  While the overall unit is probably not something you’d want to ‘F’ with, the limiter component could be added to a DIY tube mic preamp (perhaps with a true-bypass switch).  One caveat: as the article warns, there are apparently ‘thumping’ artifacts inherent to this circuit, which was acceptable in its intended operation since it was designed for voice-frequency.

1954_limiter_SchemAnyhow, might be a fun project for a rainy day – build a pair of these into box with cheap 1:1 input transformers and simple line output stages?  Crush some room mics?  Anyone?

Boozhound Labs Phono Preamp Kit

IMG_0426I know.  The grey box above ain’t much to look at in the abstract.  But, it’s what’s inside the box that we are concerned with today.  Jsn at Boozhound Labs (hf ‘BHL’) sent me one of his new “JFEt Phono Preamp Kits” to check out, and I’m glad I did.  It was a quick+ easy project that has greatly improved my LP listening situation in the living room mini-system.

IMG_0455Above you can see the BHL preamp right beneath a little stereo power amp (with A/B input select and a stereo volume pot).  The power amp is a design that I have built dozens of times for clients: a single 6SN7 is shared to provide one voltage-gain stage per channel, with each channel using a 6L6 (cathode biased) class-A to provide approx. 8 watts of power to each speaker.  Just so you know where I’m coming from, here’s the complete setup:  ADC QLM 30 mk III cartridge mounted on a Technics 1200 table, then into the BHL, then to the SE 8-watt power amp, and finally to JBL 18ti speakers.  Very circa-1980.  The other input of the power amp is connect to an Apple Airport Express so that I can stream music off the iPhone or the macbook.  Certainly not an audiophile setup, but I’ve never found it lacking.

IMG_0425The BHL kit ($89 direct) comprises the board that you see above, all the parts to stuff the board, instructions, and a bunch of good quality wire.  I supplied the Hammond steel chassis and bottom plate and the vintage-style lamp holder and big power switch, plus a $13 24v switching power supply from eBay, some El Cheapo-brand dual RCA jacks, a binding post, and a goofy old Amphenol connector to mate the power supply.

IMG_0427Note that since this thing is running at 24V, I had to try to find a 24v bayonet bulb for the vintage-type jeweled-lamp.  Don’t use a 6.3V bulb from yr parts drawer here!  It will not end well.  Luckily, MCM electronics had 28v bayonet-base bulbs (the closest value that I could find to 24v) for just a few cents.  Anyhow, I went through this trouble since I wanted the BHL preamp to match the appearance of the power amp perfectly, but you could easily build this board into whatever you want.  Here’s a dude on Instagram who went for more of an ultra-modern/brutalist look for his build.  Point is, you can do whatever you like as far as the visual aesthetic yr after.   I probably spent about $50 for the non-included components for my build.

The circuit is super-simple and it’s very very easy to assemble. The BHL site has a transcript of the directions that come with the kit if yr curious.   It took me less than an hour to solder all the parts together.  As Jsn explains on his site, “This is just about the simplest circuit possible that will accomplish what we need – reverse-RIAA equalization with gain. This is 2 JFET gain stages with a passive (no feedback) RIAA equalization network sandwiched between them.”  Simple as it is, the components included are of a very high quality.

Now, I was replacing a very cheap phono preamp with the BHL (I had been using a $50 Rolls VP29), so keep that in mind – but here were my initial impressions, which the past month of 4-hour-per day listening has proven to be (subjectively) correct:

*The sound is very good; the low end is a bit more even and less ‘rolled off’ versus the ROLLS preamp.  Vocal-area midrange is a bit more forward.  The high end seems to sound about the same, but the cartridge/LP is probably the limiting factor as far as treble.

*The biggest improvement is that the self-noise of the BHL is so much lower.  And there is less hum.  So overall there is really a huge difference in terms of background noise, which just brings out so much more detail in the music.  And honestly, I never even thought that my ROLLS preamp was noisy until I installed the BHL.

*Gain is a little lower than the ROLLS that it replaced (maybe 2 or 3 dbs).  Not ideal, but not a huge deal.

Full disclosure, and maybe this is unnecessary: Jsn provided the kit to me at-cost.  That being said, at the street price of $89 I do still think it is a very good value.  If yr thinking of testing the waters of Audio DIY (and you like listening to LPs…), I could not think of a better place to start.  Jsn is a great guy, and it was his old BHL blog that was one of my greatest inspirations in starting PS dot com; so if you dig this website, support the dude and get yrself some upgraded sound in the process.

Boozhound Labs

Kinda Miss My Camaro

CamaroFuzzTopCC copyAbout 10 years ago I had a dark purple 1986 Camaro Berlinetta with a 305 and the digital ‘knight rider’ instrumentation panel.  I bought it for $775 (only 83k miles!) from an old lady who grumbled, “don’t kill yourself Don Johnson” as she handed me the keys.  Except for one odd incident in which the car would simply not turn off,  it was 100% reliable and I loved driving it.  I sadly had to sell it because I lost the parking space it slept in.  I was poking around the ole MacBook today trying to find some pics of it, and there ain’t even one!  I guess that 2005 was maybe riiiiiiiight before the era of ubiquitous photography of every single fkkn facet our goddamn lives, and maybe I’m better off for it.  Maybe Camaro is just better off living in my midnight-blue tinted memories.

Camro_Fuzz_insideWhile I couldn’t find any Camaro car pics on the laptop, I did stumble on the above-depicted germanium Fuzz Face ‘clone’ that I decorated with a mid-seventies Camaro badge.  I buy these old chrome auto badges in lots at the flea market; if yr patient, you can get em for a buck or two each.  Anyhow, this is one of the only guitar pedals that I ever built and it turned out great, despite the fact that I simply used whatever old NOS RCA germanium PNP transistors I had around, without so much as even looking at the data sheets.  I just tried a few different types until the thing sounded like a Fuzz Face, and voila.  One cool detail: in the image above, check out the massive mil-type chassis-mount oil cap (far left) that I used as the output coupling cap.

fuzzfacepnpschematic copy
Above is the very simple schematic that I apparently pulled from FuzzCentral.  Much like my Camaro (the car), my Camaro (the fuzz) was sold off many years back, and yeah I kinda miss it.  Can’t keep em all…

Federal AM-864 Limiter Clone: Build Notes: update 1

Federal_AM864_clone_w_mic

Note: I performed extensive frequency, level, and actual studio tests on the 864 clone today, and several interesting details were revealed.  Text has been edited to reflect that. 

From 1954 through at least 1963, the Federal Television Corporation built an audio limiter called the AM 864/u for the US Air Force and US Army.  The 864 is a simple, rugged device that accepts 600 ohm balanced or unbalanced line-level signal, offers a single front-panel input-attenuator control, and compresses the output level at a 10-to-1 ratio once the threshold point is reached.  The output is also 600-ohm balanced or unbalanced, and it offers a maximum 36db of gain.  The rear panel of the unit displayed the threshold and ratio controls, although these are confusingly referred to as (respectively) CURRENT and THRESHOLD in the manual and schematic.  Attack and release times are fixed, and the manual indicates them at .05″ and 2″ respectively.

AM864_hookupAs you can see in the diagram above, the 864 was intended to be used as what we call a ‘broadcast limiter’ – the final step in the signal chain before the broadcast transmitter.

Download the original 1963 manual for the AM864 (apologies to whomever did the epic work of scanning this 55pp document; I have long forgotten where I got this file from)

DOWNLOAD: Federal-AM-864-U-Manual copy

Federal_AM864_on_benchAfter having scratch-built an Altec 436 compressor years ago, I wanted to try building an 864.  The circuits are very similar, although the 864 uses the older 1940s-era octal tubes and uses a feedback circuit from the plates of the input tubes (rather than the output tubes, as in the 436) for its compression control signal.  More importantly, though, the 436 remains a bit of an oddball underdog in the vintage-compressor market while the 864 enjoys a very strong reputation.  Anyhow, like the 436, the parts cost to build one of these things is negligible, so I figured what the hell.

This is going to be a very long + detailed +technical article, so I’m going to ask y’all to please click the link below if you dare to READ-ON,,,,

Continue reading Federal AM-864 Limiter Clone: Build Notes: update 1

Suicide Manual

TAB_666_ExperimentingIn NYC in the mid-seventies, an electronic-based band arose amongst all the guitar punks, a band that was known as much for their confrontational post-beatnik vocals as for the strange and intense sounds that emanated from their famously homemade electronic sound equipment.  A band who has become, in the decades since, one of the few acts that is truly ‘required reading’ in the lexicon of avant-garde rock n pop.  Or, as James Murphy so brilliantly puts it in his apocryphal tale of musical uber-taste, “I was there, in 1974, the first Suicide practices in a loft in New York City… I was working on the organ sounds…with much patience” (skip to 2:50… or, actually, don’t… this song kinda rules).

So yeah I am talking about Suicide.  If you don’t know ’em, check ’em out…  it is amazing+terrifying that this record came out in 1977…  truly truly AOTT.  And plainly awesome too.  I really love this band, and they inspired me greatly in the early 2000s, when I was performing with a punk band in Brooklyn using an analog drum-machine rig based around some old Roland beatboxes, voltage controlled filters, and a CV-generating homemade theremin to control the whole thing.

LISTEN: The_Flesh_Gallows

This felt fairly fresh to me in the year 2001; so that fact that Suicide was doing this same thing 25 years early was mindblowing.  I had to wonder; how the hell did these guys make all the stuff?  Even in the year 2000, DIY’ing synth equipment was fairly unusual for rock musicians; but in 1975?  That was like black magic!  Well I think I found the grimoire.

NEways… kinda a long setup to what will be…  the first OUT OF PRINT BOOK REPORT we’ve had in a while.  And oh boy will there be more coming.  I was recently at a really fascinating estate-sale somewhere in Marin County, California, where I met an elderly engineer who sold me a library of ancient audio-tech books and wished me luck on my travels… the pick of the litter was the above-depicted “Experimenting With Electronic Music,” by Robert Brown and Mark Olsen.  Published in 1974, it is TAB books catalog number 666.  No joke.  This just keeps getting better.

ARP_2500The book starts with some fairly uninteresting discussion of various commercially-available synthesizers circa ’74, but soon gets into a wealth of both schematics and ideas regarding DIY’d audio electronic circuits.  Here’s the TOC:

TAB_666_ContentsThere’s a ton of great stuff in here, and while I honestly have no idea whether or not the particular transistors spec’d in these circuits are still available, I would imagine that there are subs available…  even if you never build anything from the book, I think anyone with an interest in early electronic music will find it fascinating.  Here’s a few projects that I plan to do at some point:

PhotoElectric_Modulator Tremolo_Schem BandSelect_Audio_filter“Experimenting with Electronic music” is available from a few sellers on Abe Books.  It ain’t cheap, but I’ve been digging for these sorta books for 20 years now and this is the first copy I ever came across.