A75 and adcom gfa-5400

[ctong]

I have an adcom gfa-5400 whose design I think is based on the A75. However, the front end is only half of the A75's complementary circuit. The output stage is biased AB rather than A. I have not heard the A75. What sonic difference between the two do you expect? Are there any obvious ways to improve the 5400 (e.g., increasing bias?)

Do the bigger adcom's (5500 and 5800) have full complementary front end like the A75?

[GRollins]

Saying that an Adcom is based on the A-75 is like saying that a pickup truck is based on a Porsche. Well, yes, they both have internal combustion engines, four wheels, and upholstery inside, but neither really has that much to do with the other; neither design is derived from the other's.
The complementary output follower stage is the most common output stage around. The only question is how hard it's biased: class B, AB, or A. Front ends vary somewhat more, but the most common front end design will have either one or two differentials, followed by a second stage (often called the VAS), which leads to the output stage.
As far as trying to hot-rod a commercial amp, you're pretty much limited by the original circuit unless you intend to gut the thing and use the chassis to build another amplifier entirely.
Feasible ugrades include:
--Increased bias. It's a rare case where you can take this much further than the original circuit. The available heatsinking and power supply are pretty hard limits. But if you can increase bias, it's generally a good idea, sound-wise. Speaking of wise, it's always good to remember that you can destroy your amp by pushing the bias too far.
--Upgrade caps. If you see an electrolytic, you might be able to get a film cap in...or you might not. Film caps are much larger (and more expensive) but sound better. Ceramic disc caps are obvious candidates for replacement also. As are tantalum caps.
--Increase capacitance in the power supply. This also runs into limits, both in terms of physical size (unless you intend to outboard the caps) and inrush current (which can be dealt with using varistors or soft start circuits).
DO NOT start randomly replacing active devices without thinking it through thoroughly. DO NOT start randomly changing resistors. If you want to redesign the front end, then go at it with a detailed game plan--including the contingency of going back to the original circuit if your circuit doesn't work.
Always keep in mind the dictum: First, do no harm.

Grey

[ctong]

Originally posted by GRollins
Saying that an Adcom is based on the A-75 is like saying that a pickup truck is based on a Porsche. Well, yes, they both have internal combustion engines, four wheels, and upholstery inside, but neither really has that much to do with the other; neither design is derived from the other's.
The complementary output follower stage is the most common output stage around. The only question is how hard it's biased: class B, AB, or A. Front ends vary somewhat more, but the most common front end design will have either one or two differentials, followed by a second stage (often called the VAS), which leads to the output stage.
As far as trying to hot-rod a commercial amp, you're pretty much limited by the original circuit unless you intend to gut the thing and use the chassis to build another amplifier entirely.
Feasible ugrades include:
--Increased bias. It's a rare case where you can take this much further than the original circuit. The available heatsinking and power supply are pretty hard limits. But if you can increase bias, it's generally a good idea, sound-wise. Speaking of wise, it's always good to remember that you can destroy your amp by pushing the bias too far.
--Upgrade caps. If you see an electrolytic, you might be able to get a film cap in...or you might not. Film caps are much larger (and more expensive) but sound better. Ceramic disc caps are obvious candidates for replacement also. As are tantalum caps.
--Increase capacitance in the power supply. This also runs into limits, both in terms of physical size (unless you intend to outboard the caps) and inrush current (which can be dealt with using varistors or soft start circuits).
DO NOT start randomly replacing active devices without thinking it through thoroughly. DO NOT start randomly changing resistors. If you want to redesign the front end, then go at it with a detailed game plan--including the contingency of going back to the original circuit if your circuit doesn't work.
Always keep in mind the dictum: First, do no harm.

Grey

To my eyes the front end of 5400 is almost exactly one half of the A75's. (If the Porsche had two engines then the pickup truck used one of the same).

I think it is safe to say that there are substantial differences between the two amps (the 5400 is somewhat harsh in the mids and treble and the latter is also rolled off somewhat). Would one expect this much degradation from using one half of the circuit alone or something else is responsible?

As far as increasing bias I might be able to put in a different heatsink (or a fan, ouch).

Speaking of caps, I noticed that many power amps do not use film caps to by-pass supplies, only electrolytic. Is that because it is not necessary to reduce the supply impedance at high frequencies or because of cost?

[Nelson Pass]

At the time I was re-designing the Adcom products, so you
will note some similarities. Keep in mind that the A75 was also
Norm Thagard's design.

[ctong]

Originally posted by Nelson Pass
At the time I was re-designing the Adcom products, so you
will note some similarities. Keep in mind that the A75 was also
Norm Thagard's design.

Thanks for confirming the similarities between the amps.

I just increased the output stage bias of the 5400 from about 190 mA (I was surprised to see how low it was) to about 340 mA. Now it sounds much better overall. The mids improved significantly although the treble is still somewhat rolled off. I left the amp cover open and the heat sink is still not too hot to touch. Maybe I can increase the bias more.

[anatech]

Hi ctong,
The Adcom is a pretty good amp, but increasing the bias is not the way to reduce distortion. It's an AB amplifer with overall feedback and there is a sweet spot for bias current. If you can't leave your hand on it, it is too darn hot.

I plan to do a bias vs THD demonstration at Burning Amp, you should attend. As for your amplifier, follow the bias setting procedures in the service manual.

Nelson,
Your hand is visible in many designs. Some merely inspired by your designs.

Did you get a chance to work with Walt Jung at all? I understand he was also involved with Adcom at some point in time. I'm not sure if you fellas overlapped at all.

-Chris

[Nelson Pass]

Originally posted by anatech The Adcom is a pretty good amp, but increasing the bias is not the way to reduce distortion.

On the Mosfet amps, you can reduce measured distortion by
increasing the bias, but you can't increase it by very much due
to the heat sinking.

Originally posted by anatech Did you get a chance to work with Walt Jung at all? I understand he was also involved with Adcom at some point in time. I'm not sure if you fellas overlapped at all.

We did not. I believe Walt advised them on some op-amp
based preamp circuits.

[GRollins]

There's more to it than the temperature of the heatsinks. If the rails collapse, you're going to generate more distortion and problems.

Grey

[ctong]

Originally posted by GRollins
There's more to it than the temperature of the heatsinks. If the rails collapse, you're going to generate more distortion and problems.

Grey

The supply should be able to handle more than 2A per channel (approximately 220W). I doubt that I can increase the idle dissipation to more than 30W per channel due to the size of the heat sink unless I use a fan.

[anatech]

Hi ctong,

I doubt that I can increase the idle dissipation to more than 30W per channel due to the size of the heat sink unless I use a fan.

Don't forget about the thermal fuse in the transformer.

Hi Nelson,
I forgot this was a mosfet. I'm working on a BJT version now. Duh!

I did get some of the mosfets from the new series parts kits. One day I may try a Mosfet front end.

I believe Walt advised them on some op-amp

Yes, that is correct. I was just wondering. My guess is that his project was over by the time you had an interest.

-Chris