power amp for small studio

[guitarmaker]

Hi everyone,

I'm looking to build a tube power amp for playback in my small home recording studio and am unsure of where to start. Here's my situation:

room size = 12' x 13' x 9'
speakers = Paradigm Mini Monitors (90db sensitivity)
input = line outs from a Lynx 2 sound card (up to +20dbu signals)

I was initially thinking that I'd need to make a pp amp (something like an updated Dyna Mark 3) to get enough power to run these speakers but I don't know if that's really the case?? If a SET amp would work fine and sound better I'm all for that. I guess given my sitaution can anyone recommend an amp project they think would be good to build? I've spent most of the last year making studio projects from scratch (preamps, EQ, compressors) so my soldering iron is hot and I'm ready to roll. Just need a little push in the right direction.

Thanks in advance for any advice, direction, and help.

Steve

[Geek]

Room that size, 2 watts per channel would be plenty. A SE amp based on a vertical-sweep triode would be efficient and cost effective

[SY]

For monitoring, you really don't want to use amps with high coloration, no matter how attractive that coloration might be for living room listening. That eliminates SE.

You also don't need scads of power- I'd be looking at something like a push-pull EL84 design in the 10 watt range.

[rcavictim]

I'd suggest a P-P triode or triode-wired pentode design. 6CA7's as in your Dyna ST-70 would be a good choice in either mode. I'd also ignore advice such as "2 watts is plenty" with 90 dB sensitivity speakers. I'd suggest an absolute minimum of 12-20 watts/ch. How loud do you want to listen? You will want to hear the low level details in the background and that takes dynamic range which takes power. Even with 20 watts you will only get max 103 dB SPL from a perfect 90 dB speaker (theoretical concept used for illustration only).

If you can score the output iron from a pair of Heathkit EA-3's you could make a superb sounding copy of the power amp end of that circuit using P-P UL 6BQ5's for ~14 w/ch. I obtained and upgraded a pair of these amps and they are among the best sounding tube amps I own, and superbly quiet, even on 100 db speakers. As a bonus the 6BQ5 is inexpensive and plentiful. If you look up 'tone' in a better dictionary you will see a picture of the 6BQ5/EL84.

If you can score a pair of EA-3's just do the mods I did and save time building. I will be happy to share my secrets with you if you wish. I'll even gift you a pair of plate chokes I used in my mods. This offer is made for you only and conditional that you obtain and retrofit a pair of EA-3's. Of course what you decide to do is entirely your choice.

[anatech]

Hi guitarmaker,
I agree with rcavictim, you will want more power than a couple watts. I don't think 20 - 35W / ch is out of range for you. I did a fair amount of studio audio work and they all like to crank it up.
The EA-3 amps would be good, an ST-70 (Dynaco) or HF-87a (Eico) would work well too.

Hey rcavictim,
I have one EA-3 and a EA-2. Output transformers are different I think (haven't looked in a few years). I am interested in your update. Care to share?? I will get around to rebuilding them sometime.

-Chris

[rcavictim]

Originally posted by anatech

Hey rcavictim,
I have one EA-3 and a EA-2. Output transformers are different I think (haven't looked in a few years). I am interested in your update. Care to share?? I will get around to rebuilding them sometime.

-Chris

Hey Anatech,

The EA-3 is ultralinear, the EA-2 was not. That means different gains in the o/p stage so there are probably some minor circuit changes in the driving stages too.

When you are ready to do the upgrades contact me. It was simple stuff like bypass caps here and there, a plate choke in a line stage and installing an RCA U-link to get straight into the power amp without the tone and volume circuits. I also dropped the value of the grid-1 drain resistors on the 6BQ5's to keep a tube from running away every time a cosmic ray went through it. Maybe 270K instead of 470K (I forget). It worked like magic.

BTW, My pair of EA-3's are available for sale. My standby power generator committed suicide last week and I need to raise money to replace it. PM me if interested.

[anatech]

Hi rcavictim,
Sorry to hear about your generator. Repairable or really really damaged?

I have a basement full of projects, so my wife says I can't buy anything else for now (forseeable future). I will take you up on your offer of help on my EA-3 when I get to it. Thanks!
I may be able to find an interested party for you (your EA-3's) I will put them directly in contact with you if they are interested.
-Chris

[guitarmaker]

Thanks everybody for such quick and helpful advice!

Listening levels are typically between 70 abd 80 db. How would this effect the choice of amp power? I was initially drawn to the Dynaco stuff because I see that Triode sells replacement transformers and the circuit looks pretty basic with lots of mod potential. Can't say I'm crazy about KT88s though. If I could get by with an EL84 or EL34 design I'd be happy too. Just don't know enough to be able to pair a schematic with a nice set of transformers, etc.. I'm better at copying an already established design.

Thanks again for all the great help.

Steve

[rcavictim]

Originally posted by anatech
Hi rcavictim,
Sorry to hear about your generator. Repairable or really really damaged?

I have a basement full of projects, so my wife says I can't buy anything else for now (forseeable future). I will take you up on your offer of help on my EA-3 when I get to it. Thanks!
I may be able to find an interested party for you (your EA-3's) I will put them directly in contact with you if they are interested.
-Chris

The engine ran out of oil and seized solid. It is a 3600 RPM job so it started out as junk. I am prepared to move on. I am looking for a real 1800 RPM plant, 6.5 kW or so. Preferably diesel. Yes please pass anyone interested directly to me. I have a lot of stuff I should put on the for sale forum.

I'd be happy to assist you with your heathkit amps when the time comes.