stereo amp build

[wade]

Hey guys,  Forgive me if this has been asked before. If I wanted to have two circuits in one chassis, could they share power transformers? Is it possible to make a stereo guitar amp, say, like two Champs?
Thanks

[92Volts]

Yes!

For hi-fi you might worry about signal bleeding between the channels as their activity affects and also relies on the same power supply.

And if your standards are really high or you're having trouble preventing this, it might be easier (or just "better") to use separate power supplies.

But for a guitar amp (or anything but the most expensive audiophile stuff...) you likely don't care.

One thing you should consider is single-ended amplifiers are susceptible to hum/buzz from poorly-filtered power supplies. You're drawing twice as much current as a normal Champ or similar, so you'll need a solid power supply to avoid noise. Increasing capacitor value is an obvious method, but you can also add a resistor/capacitor filtering stage: http://www.ax84.com/static/sel/AX84_SEL_101004.pdf

This same concept provides clean power for nearly all preamp designs, but Champs (and a lot of others) don't bother doing it before the poweramp.

[kagliostro]

To avoid some interactivity on PS nodes you can insert a diode in series with the HT path

this is used by some HiFi guy to avoid the discharge of preamp PS node capacitors when there is a high current request on the

Power Tube node

The same way you can have only one PS that has two separated branches (one for each amp) and insert separation diodes

to avoid interaction between the two branches

Franco


p.s.: However, this is a Capitol stereo guitar amp and as you see there is only one PT

[TIMBO]

This is a local built amp.
http://www.ozvalveamps.org/goldentone/gt_stereo_six_schematic.gif
I can't say how well it works, I have a stereo transformer set that I'm looking to build the above amp.

[silverfox]

It is challenging to employ the right effects if you are seeking a true stereo experience. There can be an advantage if you are using it as a power amp and want the bi-amp capabilities of separating the hi and low freqs or if you drive different impedance speaker cabs, which I'm currently messing around with. Each channel has 4-16 ohm tap switch. I use a stereo power amp and an emulator. Very satisfied. I did not isolate the channels with diodes and haven't had any problems but then again I don't have a preamp.

Before you locate the OT use the headphone technique to make sure you aren't getting hum transmitted from PT to OT.

silverfox.

[Steve_P]

I have a Guild 200-s waiting for caps to come in. Two cathode biased 6CA7 amps with a common power supply. It looks to me like they doubled the power supply vis a vis a single amp. The first node is 60 mfd, the second is 80, the other two nodes are 30 mfd. I need to source a couple of extension cabs and a stereo phaser to get maximum swirliness. Even though each amp drives a separate speaker, it's not as obvious a stereo effect with the combo cabinet.

[92Volts]

Sorry to jump in here, but has anybody done a dissimilar stereo guitar amp? Like one side for most of the power and a smaller output section for reverb only (for example)?

I've been considering driving a reverb tank off the main OPT which looks like it "should" be simple, then feed it to something small and simple which drives another speaker. Of course some reverb tanks have an 8 ohm impedance and I doubt they want 1/2 my 80 watt amp's power! Will an 800 ohm model work OK driven by a speaker-level signal?

I know the obvious downside is "wasting" an expensive and bulky second speaker which could help handle the main power, or a true double-power stereo amp. Not really sure I'd go through with it, just dreaming up uses for all these oddball parts I have!

[wade]

Thanks guys, Good to know there are possibilities.  I'm thinking a stereo head with two speaker cabs.

[silverfox]

I've been quite pleased with my Sunn inspired, (really a Dynaco 70 watt version), S100 Webber schematic inspired design. Since the TW OT have 4-16 Ohm taps I can gang my speakers in different configurations and use one channel as a PA and the other for guitar or whatever. Been doing Karaoke lately.

While I haven't encountered any channel crosstalk, the diodes on the plate supply is something I should have done. Personally, for versatility I wouldn't incorporate a particular preamp. You can always build that separate.

silverfox.