A12 vs. J99

[drunton]

Any comments on the sound "color" difference between the A12 and the J99.

I have transformers and a 2520 opamp to build an A12, but was trying to decide on 2 J99's or 2 A12's for a couple other slots.

The simplicity of the A12 is interesting compared to the relative complexity of the J99. Both are audio chain capacitor-free, if I use relatively color-less transformers like (JT16 for input) and a 990 opamp on an A12 board, is there much to be gained from the twin-servo design of the J99?

[tpryan]

It's a good idea, but unfortunately it's not quite that simple. The JT-16 is a 1:2 ratio input transformer, and it's intended for use with an amplifier that needs a low source impedance, like the 990c or AD797. However, the A12's input pad, feedback, and gain selection resistors are all calculated for an op-amp like the 2520, which has significantly lower input bias currents than a 990c. If you drop a 990c or AD797 into the A12, there's a good chance you'll see some odd things happening at the output pin, since there won't be enough feedback current to stabilize the op-amp.

One way to build a clean, uncolored version of the A12 without changing resistors is to use a Jensen JT-110K-HPC on the input, JT-123-ALCF on the output, and the OPA627/BUF634 module. If you want a discrete op-amp, a Millennia M99 or the Forsell parts would be a good choice.

[drunton]

Thanks Tim, now I understand that it's not quite as simple as swapping transformers and opamps. I would have to also redesign all of the resistor values.

But, it could be done, just through comparisons to John Hardy's M1 schematics.

Oh, and a lot of trial and error (ie: oscillators are just poorly designed amplifiers), design at your own risk, etc. etc. Sounds like a fun DIY mod to the ultimate DIY preamps!

[Category 5]

One way to build a clean, uncolored version of the A12 without changing resistors is to use a Jensen JT-110K-HPC on the input, JT-123-ALCF on the output, and the OPA627/BUF634 module. If you want a discrete op-amp, a Millennia M99 or the Forsell parts would be a good choice.

Tim, Can one simply substitute these two transformers without making additional modifications to the circuit? In other words, are they a direct replacement for the Cinemag and Profile? What about the input transformer jumpers.

I am making 2 A12s to be as close to API as possible, but I was thinking that using these transformers with a Hardy 990 or Millennia 990 would make a great M1 like, or Millennia like clean preamp. Is this correct? If so, it'd be great for acoustic instruments, overheads, etc. I'd love to build the 990 kits you offer but after buying transformers and dual op-amps for each chnnel they can get expensive...the modified A12 could be a cheaper alternative for a clean gain design.

Shane

[tele_player]

Just in case you haven't got a clear estimate of how much J99 costs with 'all the options', I built two, and they came to about $475 each. Hardy 990c opamps, Lundahl input, Jensen output, Jensen isolators. They are excellent, but they're about $100 more than the A21 (figuring $75 for an Avedis).

I just started an A21, 2 hours done, almost ready. It's MUCH simpler than the J99.