After much toil the beast lives!! This is the first prototype assembly of my 9 band (1 octave / band) all tube active EQ (codename TEQ-9B). This is partially based on the very first gyrator based tube EQs from the late 50's whereby each band is handled by a discrete tube gyrator providing +-12dB of boost/cut. All five 12AX7 tubes very much play an active role in shaping the sound - ie. this is not simply a passive EQ with tube output buffers.
I have taylored the EQ for use in studios specifically as a colouration device for use with single sources. It will be housed in a bespoke 2U rackmount chassis and features balanced I/O on both XLR and TRS jacks with an overdrive facility to permit the EQ stages to be pushed into as much non linearity as you wish 🙂
Here is a sneak preview of the first bare pcb assembly in action:
Download D.A.Wilson_TEQ-9B_Active_9_Band_Tube_EQ_Demo.wav from Sendspace.com - send big files the easy way
All A-B tests: A = bypassed audio B = EQ'd audio. Please note that in bypass mode two triodes in the output amplifier and the NE5532 based preamp and balanced line drivers are still switched in.
I have taylored the EQ for use in studios specifically as a colouration device for use with single sources. It will be housed in a bespoke 2U rackmount chassis and features balanced I/O on both XLR and TRS jacks with an overdrive facility to permit the EQ stages to be pushed into as much non linearity as you wish 🙂
Here is a sneak preview of the first bare pcb assembly in action:
Download D.A.Wilson_TEQ-9B_Active_9_Band_Tube_EQ_Demo.wav from Sendspace.com - send big files the easy way
All A-B tests: A = bypassed audio B = EQ'd audio. Please note that in bypass mode two triodes in the output amplifier and the NE5532 based preamp and balanced line drivers are still switched in.
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It will be a finished unit housed in a 2U custom rackmount chassis which is currently in fabrication. The prototypes require calibration and a significant degree of auditioning and field testing before release.
Despite it being aimed at the studio gear market, there has been some suggestion that guitarists may also show an interest in this unit. The EQ does indeed sound very sweet with bass and lead electric guitar.
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A quick addition to the basic spec:
The heaters rails are regulated DC and the anodes are fed from a regulated 220 Volt B+ rail - ie. not a starved plate in sight! 🙂
The heaters rails are regulated DC and the anodes are fed from a regulated 220 Volt B+ rail - ie. not a starved plate in sight! 🙂
I happen to have a couple of questions:
Is this thing mono? Well I think it must be, since it seems to use gyrators with ECC83 and R-C networks, and there's only five 12AX7/ECC83. Also the pots (log/lin?) look like mono. What about the tenth triode system, is it used to generate the output signal by mixing the input with the eq'ed signal?
What do you use for an input-differential, opamps? I hope not...
Or is the cicuit more like this one: http://www.triodeel.com/images/b9b.gif?
What does the 11th pot do?
Why are there some electrolytic caps so suspiciously close to the XLR connectors?
And finally, how do you keep the circuit's output balanced? Opamps?
Is this thing mono? Well I think it must be, since it seems to use gyrators with ECC83 and R-C networks, and there's only five 12AX7/ECC83. Also the pots (log/lin?) look like mono. What about the tenth triode system, is it used to generate the output signal by mixing the input with the eq'ed signal?
What do you use for an input-differential, opamps? I hope not...
Or is the cicuit more like this one: http://www.triodeel.com/images/b9b.gif?
What does the 11th pot do?
Why are there some electrolytic caps so suspiciously close to the XLR connectors?
And finally, how do you keep the circuit's output balanced? Opamps?
So Many Questions 🙂
Yes - the unit is mono, as it was aimed at processing single sources prior to mixdown in the studio eg. bass, vocals etc as a means to add colouration rather than it being a super accurate mastering device. There is a dual channel unit in the pipeline.
The gyrators pass the full audio signal ie. there is no mixing of dry and wet signals.
The electrolytics are there to decouple the rails (there are no electrolytics in the audio path) for the op-amps used to implement the balanced line I/O using NE5532's (a popular choice in studio gear).
The 5th triode is used to implement a post-amplifier and buffer stage, as in the B9B, for gain control and buffering as the mixed output from the gyrators is very high impedance.
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That's a little cruel - this started as a project for something I wanted in my studio and since it didn't exist (the market is awash with tube parametrics but you try and find a genuine active tube GEQ) I decided to design my own. What has happened since is that a number of other musicians and studio owners have said they'd like one too.
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If I understand your description correctly you're using tubes (a bunch of them !) in the equilizer section because of their imprint on the sound, yet you resort to opamp (NE5534 or something like that) for preamplifier duty ?
Seriously ?
The NE5532's are there specifically for low gain balanced line I/O buffering which is both transparent and low noise. 5532's are used in countless pro-studio gear designs and are quite well regarded in these circles in such applications.
They are there to offer a low impedance drive to what is otherwise a very high impedance tube stage which would otherwise be rather susceptible to noise. The track lengths between the I/O buffers and the tube stages have been kept as short as possible for this reason.
I don't mind the use of opamps (I'm not one of those "this type of device is the only one useful and the rest are plain dreck" kind of people) but it sounds odd that you have chosen tubes for the equalizer section because of their impact on the output, whatever that might be, yet you omitted them in the pre-amp section, apparently due to the imprint they might leave on the sound output (this is how I understand your reply quoted above).
Why not add one extra tube, get rid of opamp and call it a day ? Anybody who buys this thing because of its tubey appeal will appreciate the "full-tube complement" and those who want pure unaltered cold sound will loathe it anyway due to the number of tubes in the EQ section.
You've got a great tube product there, yet you slapped the opamp right in the middle of it as if tubes have somehow lost your vote of confidence after you used them 9 times ... If there's a good reasining behind it I'd sure like to hear it 😉 What you posted above didn't convince me.
The unit doesn't feature a preamp as such - it was designed to be run at instrument levels. The op amps are only used in the near unity gain balanced line I/O - mainly because to implement high quality, low noise tube based balanced input and output stages would require the use of transformers which would make an already expensive design prohibitively expensive. To implement transformerless balanced line in tubes is pretty darn tricky. We take op amps for granted these days but this is one of those tasks op amps do very well indeed.
If the unit were designed with single ended I/O for the hi-fi market then, as you suggest, I probably would have made it all tube but running single ended in the studio is just not an option as the cable lengths typically involved would soon overwhelm the tube stages with a sea of mains hum.
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