Two-channel tube/hi-fi blending preamp - diyAudio
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Old 12th November 2010, 08:05 PM   #1
diyAudio Member
Join Date: Oct 2010
Default Two-channel tube/hi-fi blending preamp

I'm trying to settle on basic schematics for a 2-channel blending preamp build for my active pickup extended range bass.

First channel would be a switchable dirty/clean tube preamp, the second, a discrete or opamp hifi stage. Each channel would have its own eq, & its own buffered effects loop. The blending would be controlled by a pedal potentiometer or (better yet) a pedal remotely linked to an internal potentiometer, so that no signal actually goes outside the preamp/effects rack.

Concept being, instead of a instant footswitch-controlled change from say, clean rhythm guitar to overdriven lead, you could do either, or any infinite variation between. Have two distinct tonalities in the eq's and sweep between them for a "wah" effect. The tonal possibilities really would be quite extensive.

Haven't decided if I would place a limiter/compressor at the very output (to help equalize the levels of the two channels) or on each channel individually.

I've already considered something along the line of the matchless hotbox for a tube circuit, and there's several good hifi designs (several of the opamp preamps on Rod Elliot's site, or the Kreuzer Bass preamp, looking at something more discrete). Problem is, I obviously want to the circuits to be somewhat compatible gain-structure and phase-wize, and I also want to try and keep the power supply requirements simple as possible. I'm already building up Jeff's +/-15v LCBPS from the Twisted Pear site, and would like to use it (or something of comparably high quality) as the power supply for this build. I realize this might not be possible, especially if opting for a non-starved-plate design for the tube channel.

Anybody have thoughts on some diy designs out there that would be (or could be easily made) compatible? For either the tube or the hifi stage?

The kicker is if this works for two channels, I may end up building an additional two for the bass side of my NS/Stick.
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Old 13th November 2010, 01:21 AM   #2
diyAudio Member
Join Date: Apr 2010
Plus minus 15v gives you 30v. Using a simple voltage multiplier say six stages giving you about 150v. 12AU7 might be fun to use.
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Old 13th November 2010, 01:36 PM   #3
diyAudio Member
Join Date: Apr 2010
Or you can take a small 12v wall adapter and use the transformer, maybe a 12v 300mA, and put the secondary on one of your 15v power supply's transformer. A 12v adapter will put out about 9-10vac so from 120vac it has a ratio of about 13:1. Given maybe 12vac from the 15v supply you might get something in the order of 150vac and then rectified 200vdc. Mind you these are rough numbers.

Just saw your power supply and I see it is regulated so voltages comming out of your transformer will be higher. Not a bad thing.

Last edited by Printer2; 13th November 2010 at 01:41 PM. Reason: Looked up your power supply
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Old 13th November 2010, 07:06 PM   #4
Minion is offline Minion  Canada
diyAudio Member
Join Date: May 2006
Location: Cowican Bay , vancouver island
If you go to any Lighting store you can get small 120v/120v isolation transformers for just a few bucks which will give you about 170v DC or if you use a Voltage doubler get close to 340v DC , good for most audio tubes but you will still have to come up with 12v 0r 6.3v for the heaters .....
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Old 23rd November 2010, 10:12 PM   #5
ChrisA is offline ChrisA  United States
diyAudio Member
Join Date: Jan 2008
Originally Posted by Minion View Post
If you go to any Lighting store you can get small 120v/120v isolation transformers for just a few bucks which will give you about 170v DC or if you use a Voltage doubler get close to 340v DC , good for most audio tubes but you will still have to come up with 12v 0r 6.3v for the heaters .....
Cheapest way to power a preamp tube: Get two small 12V transformers. Perhaps salvaged from 1A plug in power cubes. Wire the two 12V "secondaries" together. Also use this 12V to run the heaters. You can recifiy and regulate the 12VAC for DC heaters or leave it AC. May of the cubes have rectifier and filter already soldered to the transformers. I used one to run the headers and upgraded dioodes and caps on the oother for a B+ supply, The little PCB worked fine after swapping to higher voltage components

You will get close to AC Mains voltage in the second transformers, after rectifies and filters you can have about 160VDC. that is enough to run any of the 12A*7 tubes.

I built one of these but they do take up some room with the two transformers.

Total cost: About zero.
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