Mono preamp with stereo power amp - please help

[octavedave]

Hi there,

Greetings from London, UK. I am new to this forum, and not an experienced amp builder. Although i am keen to start. I have recently repaired and modified a Mesa Studio 22 - which i am now loving.

This may sound a bit crazy, but i am keen to make a combo tube amp that has a mono preamp stage (as standard) outputting as a jack on the back (FX loop send) but also 'normalled' to a stereo power amp stage (this could be switchable to a double powered mono stage). The input to the power stage would affectively be FX Loop stereo return. The idea behind this design being that you could put some nice mono to stereo FX pedals after the preamp stage (such as stereo chorus or delay). No need for the preamp stage to be stereo, as your source (guitar) is mono. I was thinking of a bedroom/rehearsal amp in a 2 x 10' cab (20W?)

Has anyone thought about this kind of thing before?
Can anyone think of any design problems this idea?
Most importantly, are there any pre-existing designs or schematics I can use? I see that Hoofman has a stereo power amp product.

Any advice would be greatly appreciated.

Thanks in advance for any help here.

Dave

[octavedave]

Any thoughts or help with this would be greatly appreciated. Thanks

[sluckey]

The idea behind this design being that you could put some nice mono to stereo FX pedals after the preamp stage (such as stereo chorus or delay).

Most (not all) store bought pedals are expecting instrument level input and produce instrument level output. This could be a problem for your design.

[octavedave]

Thanks for you reply. Good point, forgive my ignorance, but aren't most amp fX loops running at levels similiar to instrument levels anyhow? With my Mesa Boogie studio 22, for example, I run some pedals before the amp (wah, compressor, tuner) and the rest in the fx loop (tremolo, delay, modulation). The only differnce here, would be that at the point of return the amp circuitry would become stereo.

[sluckey]

I bet your guitar would sound very weak if plugged directly to an FX return jack.

[vibrolax]

My old guitar teacher had a local amp builder make him a "double deluxe", that had two separate Fender Deluxe Reverb power sections (22W), each driving a single 12" speaker (or maybe it was 2 x 10's.  But it also had 2 complete preamps: a solid state one for acoustic guitars, and a blackface fender for electric.   It was wider and heavier than my Vibrolux Reverb.  He got tired of lugging it around, and parked it in the cupboard.

Given that one wouldn't obtain much stereo separation in a single combo cabinet, I'd think a more effective solution would involve a separate combo amp/speaker amplifying the other post-effects channel.  If you have (or could borrow) a second amp, you could test out this concept to see if it's worth anything with the speakers right next to each other, or separated.

For noodling at home, I used to enjoy the stereo output of my Alesis Quadraverb GT piped into the my stereo system (the GT had a decent cabinet simulator).  For playing out though, I didn't find the benefit of stereo effects compelling.

[jjasilli]

a stereo power amp stage (this could be switchable to a double powered mono stage).
If I understand, this is called power amp "bridging".  This is a complex topic and maybe dangerous if done incorrectly.  It would be easier to keep the 2 power amps separate, with each driving its own speaker.

The input to the power stage would affectively be FX Loop stereo return.
Building on Sluckey's points -- either:

a) use effects that are designed to be used in an fx loop; or

b) run the pedals' 2nd stereo-out through a line driver to the 2nd power amp; or into a gain stage in the second power amp.  Either way you'll need a vol/gain pot to balance the output volume of the 2 power amps.

To get true stereo from stereo pedals requires 2 power amps, feeding 2 speakers which are suitably separated from one another.  If the speakers (drivers) share the same enclosure, the stereo affect will be minimal.  The amps can share the same chassis; but this will be heavy.

a bedroom/rehearsal amp in a 2 x 10' cab (20W?)
A trumpet produces 1W of audio power.  The equivalent of 20 trumpets may not be a bedroom solution. 

[John]

The equivalent of 20 trumpets may not be a bedroom solution.

Well.... depends what you're doing in the bedroom. 

[sluckey]

...
https://www.youtube.com/watch?v=Gktb1yvjsjA

[Willabe]

 

[PRR]

> A trumpet produces 1W of audio power.  The equivalent of 20 trumpets may not be a bedroom solution.

Electric Watts are NOT acoustic Watts.

20 Watts electric, into a loudspeaker, is more like 0.2 acoustic Watts. (1% efficiency, 5%-0.02%)

[octavedave]

a stereo power amp stage (this could be switchable to a double powered mono stage).
If I understand, this is called power amp "bridging".  This is a complex topic and maybe dangerous if done incorrectly.  It would be easier to keep the 2 power amps separate, with each driving its own speaker.

The input to the power stage would affectively be FX Loop stereo return.
Building on Sluckey's points -- either:

a) use effects that are designed to be used in an fx loop; or

b) run the pedals' 2nd stereo-out through a line driver to the 2nd power amp; or into a gain stage in the second power amp.  Either way you'll need a vol/gain pot to balance the output volume of the 2 power amps.

To get true stereo from stereo pedals requires 2 power amps, feeding 2 speakers which are suitably separated from one another.  If the speakers (drivers) share the same enclosure, the stereo affect will be minimal.  The amps can share the same chassis; but this will be heavy.

a bedroom/rehearsal amp in a 2 x 10' cab (20W?)
A trumpet produces 1W of audio power.  The equivalent of 20 trumpets may not be a bedroom solution. 

Agree with you points about integration of pedals and about keeping power amp as separate circuits driving each speaker, makes total sense.

Obviously keeping speakers in separate cabinets is best for stereo separation, but if I was going down that route I may as well keeps amps separate as well - but this ends up being a big rig. Stereo combos have been manufactured before with mixed success, Roland Jazz Chorus, Peavey Classic Chorus and the newer Blackstar IDs spring to mind. You do get a sense of the stereo imaging, even if not optimal. With my design, I was toying with the idea of placing speaker either side of a central amp, ie a 'wide and low' enclosure.

However the key point you raised here was about the weight. This is a big concern and could be the deciding factor. I am guessing design will require at least 1x extra transformer and 2x extra power tubes?

Stupid question- is there a design of power amp that only uses 1 tube?

[vibrolax]

Sure.  A single ended amp can have just one per tube.  A fender champ uses a single 6v6 to give 5 watts.  An el84 can do about the same.    Two single ended power amps will give about 1/2 the power of a push-pull amp with the same power tubes.  That is, you'll get 10 watts total from 2 single ended el84 amps and 18 watts from a push pull amp with 2 x el84's.

For bedroom use, either would be overkill.

[shooter]

is there a design of power amp that only uses 1 tube

Yup, look up "single-ended class A"

I built a stereo SE KT88 *champ* style amp, 50lbs!  I gave up tuning it, now it's part of 3 new builds 

[jjasilli]

Note that a single-ended amp needs a much heftier OT than a PP amp, thereby detracting from or negating any weight savings with just one power tube.


Another possible option is to run the second channel into the house PA at a venue. 



Stereo rigs present significant practical problems and compromises for those of us without road crews and sound crews.  Also, without a dependable sound guy, it's impossible for the onstage guitarist to know how a complex rig actually sounds to the audience.

[Ritchie200]

IF you are going to pursue this, you can use this one with relatively cheap parts (to give you an idea) - just double it up.  You can build one and tweak your preamp to suit, then build the other.  This one will give you about 10-12 watts per channel.  Plenty for bedroom and miced on stage if needed.

OR you can just double up the output of a Champ circuit for even less power.  Lots of options here. Just depends on what you are trying to accomplish.  Marshall has/had the 9001, 20/20 and the 50/50, so this is not a new concept.  It is not one of their bigger sellers and I see them for sale used all the time.  They sold a few when they first came out during the rack crazy days of the late 80's.  The reason I think was verified by Vibrolax.  These amps make some noise and live stereo results can be minimal, and too loud for the basement/bedroom.

The bridging of channels is common on solid state - not tube.  You would be better off running the same mono signal into both channels if you want that option - just two different return jacks with the "stereo" jack splitting the inputs when you plug in.

This would be a fun project but is pretty ambitious for your first rodeo.  However, if you take it one step at a time, you will be ok.  Plenty of folks around here to help you out if you hit a bump in the road.

Jim

[octavedave]

IF you are going to pursue this, you can use this one with relatively cheap parts (to give you an idea) - just double it up.  You can build one and tweak your preamp to suit, then build the other.  This one will give you about 10-12 watts per channel.  Plenty for bedroom and miced on stage if needed.

OR you can just double up the output of a Champ circuit for even less power.  Lots of options here. Just depends on what you are trying to accomplish.  Marshall has/had the 9001, 20/20 and the 50/50, so this is not a new concept.  It is not one of their bigger sellers and I see them for sale used all the time.  They sold a few when they first came out during the rack crazy days of the late 80's.  The reason I think was verified by Vibrolax.  These amps make some noise and live stereo results can be minimal, and too loud for the basement/bedroom.

The bridging of channels is common on solid state - not tube.  You would be better off running the same mono signal into both channels if you want that option - just two different return jacks with the "stereo" jack splitting the inputs when you plug in.

This would be a fun project but is pretty ambitious for your first rodeo.  However, if you take it one step at a time, you will be ok.  Plenty of folks around here to help you out if you hit a bump in the road.

Jim

Thanks Jim, for both the advice and encouragement. Just wondering if this might be a bit too ambitous for my first build. Perhaps i should build a standard champ first.

If I were to bit the bullet and go for a stereo amp based on a classic Champ 5f1 design, the bit that confuses me is where and how to add the FX loop. I guessing it should be after the 2nd 12ax7 stage, and would obviously need to buffered and level adjusted in and out - perhaps with solid state OPAMPs. But i like the idea of this, perhaps feeding 2 x 8" weber alnicos.

If i went with your HiFI schematic (x2) along with a separate preamp, I am guessing that no additonal buffering would be required. Would such a 'hifi design' work well as a guitar power amp? I guess no reason why not. Thanks again