Lets talk about Op Amps.....

[s.d.finley]

I am mainly interested in building an Eisen Audio DIY 500 series pre amps.
There are quite a few different options available for the opamps, but I am mainly interested in the NEW ones.

Here are the contenders....

Hardy 990c
Avedis 1122
JFET 992-993
JLM Audio Hybrid
Millenia MM99-990
Purple Audio KDJ4
SCA SC25

So, I am wondering if anybody can give me some info on these. I realize that the input and output trannies will also have a major effect on the color of the pre.

My considerations on those are...input, some type of Sowter. Output, some type of Luhndahl.

I have come to these conclusions based on what I already have in my current Lunchbox: 2 OSA MP1C's, A-Designs P1 and EM Silver, Shadow Hills MONO Gama. Plus I am in current negotations for a 512c. So, I want something different. Plus I think this will be a fun learning experience.

Thanks ALL!!

sdf

[s.d.finley]

Anybody?
40 looks but NO responses? Am I smoking too much crack?
Should I consider different tranny options?



sdf

[matta]

Hey sdf,

Based on what you have in your rack already there a try something like a Luhndahl input trano with one of Fred's JFET 992-993? I imagine you would end up with a nice neutral sounding pre with a great transient response and something quite different from what you already have.

Hope that helps.

Cheers

Matt

[axisdreamer]

What I have found is the input and output trannys make more difference in the sound then the op amp used most of the time. I have a bunch of OSA prototype pre's with different parts on them that I use all the time in my studio.Melcor op amps sound a little dark but with a UTC 0-8 or 0-12 input tranny the pre gets bright again. The input tranny really seems to make a huge difference. I would like to try a Lundahl input,something I havn't done yet.I worked with Dale a few years when he first started OSA and that's how I found out about the tranny's influence on the sound. I have old ADM,Melcor,MAP,API op amps on mine. I never tried any op amps on your list but I know the 990 is a little too clean if you want to keep the API kind of sound.

Stace

[brianroth]

The loading on the secondary winding can make a TON of differenc on the perceived sound. That's why frequency response at high frequencies (and/or square wave testing) is important to help "tune out" gross errors, such as excessive HF boost/overshoot.

For the test to be valid, you also need to drive the xfmr. from something like a 200 ohm source.

Bri

[s.d.finley]

I guess I should've titled the post "Lets talk about Transformers"

Thanks for all the input guys.

So if the input tranny imparts a larger impact on the source than the output T or the opamp, maybe I should go with the Lundahl LL7903.

For the output, I guess I need something clean with low distortion and Hi output.
Asfaras the opamp, just lots of headroom and low distortion.

I guess I could also check out the used market for output Transformers for something different from the new market.

Thanks again!

sdf

[NathanEldred]

I've been told by people such as Tim Farrant, Andrew Roberts, Steve Hogan (Steve is actually the guy who developed the 990c), and Fred Forsell that just slapping any old transformer on any old op amp is a bad idea for various reasons. Did I hear wrong?

[axisdreamer]

I would do the Lundahl LL7903 for the input,it has a sweet hi end and huge bottom. I heard it at OSA when Dale got the first batch of them in.I want to get one myself. I also really like the UTC 0-8 or 0-12 input tranny,it has an open hi end that is bright but not harsh,they came off some ADM modules that I have.

I could sell you some to try if you want. They are what I have on most of my OSA pre's and they sound great.

Price $50.00 each

The Lundahl was over $100.00 the last time I checked.

I have some MCI output trannys that work in the API style pre.I have one with it on and it sounds good.

$30.00 each,

I have so many of them that I'm passing the savings on to you!

I also have some Yamaha op amps that sound great on these pre's.

I sell them for $40.00 each and they sound just as good as the api 2520.

I am looking to get one or two Lundahl input trannys so I need to sell some stuff off .
Let me know if you want anything from me.

Stace

[gevermil]

How do those adm op amps sound in an api type circuit ?
Ive played around with the 990c and jlm99v , both good .
Whats the 0-12 primary/secondary impedance ?
gary

[Tim Farrant]

Originally Posted by NathanEldred

I've been told by people such as Tim Farrant, Andrew Roberts, Steve Hogan (Steve is actually the guy who developed the 990c), and Fred Forsell that just slapping any old transformer on any old op amp is a bad idea for various reasons. Did I hear wrong?

No Nathan, you have a good point. The input transformer may require compensation to avoid nasty frequency response rises at higher frequencies which may cause instability problems at high gains. I am not sure if Eisen Audio gives you the correct component values relative to the transformer you choose.

Tim

[NathanEldred]

Originally Posted by Tim Farrant

No Nathan, you have a good point. The input transformer may require compensation to avoid nasty frequency response rises at higher frequencies which may cause instability problems at high gains. I am not sure if Eisen Audio gives you the correct component values relative to the transformer you choose.

Tim

Thank you Tim.
By instability you mean oscillation? What are some of the negative results that could happen by haphazardly putting pieces together? This applies to the output transformer also?

[s.d.finley]

All good points to make guys...

I thought once I have decided on the 3 main components, I would give Eisen a call. Of course I would need the spec sheets on all 3, and purchase the other parts, resistors caps, et al...later...after consulting Eisen.
They have several schematics posted on their site, too. The standard stuff...312, 512 stuff, and a few others...

I could just start with plans that they have posted....but that would be no fun

Keep the Good advice comin!!

thanks

sdf

[Tim Farrant]

Originally Posted by NathanEldred

Thank you Tim.
By instability you mean oscillation? What are some of the negative results that could happen by haphazardly putting pieces together? This applies to the output transformer also?

Yes, oscillation, probably only at high gains however.

Depending on the type of steel used in the output transformer, the low frequency response can be affected if the drive impedance is high. I can't really tell what will work and what will not, but the interaction between the chosen opamp and transformers is a real thing, and only testing of the combinations will tell you.

Tim.

[s.d.finley]

Originally Posted by Tim Farrant

Yes, oscillation, probably only at high gains however.

Depending on the type of steel used in the output transformer, the low frequency response can be affected if the drive impedance is high. I can't really tell what will work and what will not, but the interaction between the chosen opamp and transformers is a real thing, and only testing of the combinations will tell you.

Tim.

Very good, thanks for the heads up Tim!!

In this case, I shall contact Eisen and see what opamp/output trannys that they have tested with good results.




sdf

[s.d.finley]

So Ive been emailing Jens @ Eisen and he said that the 7903 wont fit on the board without some modifications, so instead I am opting for the LL1538XL, and using the JFET 992.

Jens said I could use the JFET 992 w/o an output tranny, as that is how it is designed. I decided to go that route cause of the potential problems suggested by B Roth and T Farrant..et al. Thanks Guys!!

So this will eleminate those problems. Yes, minus the output tranny will give me lower output. So instead of a -20db pad, I will go for -15db. I mainly record LOUD music, so no biggie here....

I have priced all componets, and including the $20 bucks and the slap on the back I will give my assistant for assembling the pre, total cost will be around $310!!

Now that aint shabby!!

Once I get it all together, I will post some pics and my thoughts...

Thanks for all ya'lls suggestions!!

sdf