The Awesome Cordell Amp!

[myhrrhleine]

There's a really great amplifier, designed by Bob Cordell.
It's a FET input and MOSFET output.
a description can be found at:
http://www.cordellaudio.com/papers/m...rrection.shtml
I'd like to start a discussion of updating and building this amp.
It's only 50 watts as originally designed, but it should be easily modified to over 100watts just

by raising the supply voltage to 50 volts on the MOSFET outputs and maybe paralleling 2 of them.
(+raising the input supply to 65volts)

I'm not posting this merely to share my own adventures, but rather (i hope) to get a large participation of experiences and ideas.

[myhrrhleine]

for starters, we might consider the transistors used:
Q1, Q2 LS844 Linear Systems
Q3, 14, 15, 16, 22, 24 2N3904
Q4, 5, 19 2N5550
Q6, 7, 8, 9, 10, 11, 23, 25 2N3906
Q12, 13, 21, 27 KSA1381 Fairchild
Q17, 20, 26 KSC3503 Fairchild
Q18 2N5401
Q28 IRFP240
Q29 IRFP9240 Fairchild

[syn08]

Originally posted by myhrrhleine
There's a really great amplifier, designed by Bob Cordell.
It's a FET input and MOSFET output.
a description can be found at:
http://www.cordellaudio.com/papers/m...rrection.shtml
I'd like to start a discussion of updating and building this amp.
It's only 50 watts as originally designed, but it should be easily modified to over 100watts just

Bob's Hawkford EC output stage, modified for 100W/8ohm and 200W/4ohm, lateral MOSFETs, trimmed to 80ppm:

http://www.synaesthesia.ca/OP-stage.html

PCB Gerber files are also available (click the link).

Listening test (with the PGP front end):

http://www.synaesthesia.ca/listening.html

[myhrrhleine]

cool
another step towards a working amp.

[CBS240]

Hi

I find it difficult to get a perfectly stable bias current between start up and full temperature. With 200mA bias, I was able to get the bias fairly stable when the HS temp warmed slightly and for when it is hot, but the initial bias is a bit high. I don't think this is a problem if it is not too excessive, but others may have other opinions. I didn't dwell too much on the issue. I think Bob mentioned somewhere that he only placed one of the EC transistors on the heat sink and had better tracking results. The challenge is to get the EC transistors off the heat sink. My thoughts were to use a small signal SOT-23 device and mount it on the PCB inderneth the output transistor pins, so as it contacts the drain pin right were it enters the component. I've also contemplated using a daughter board for the EC amplifier.

As for the operation of the output stage, it has great potential to get real results from fairly cheap components.

[Bob Cordell]

Originally posted by CBS240
Hi

I find it difficult to get a perfectly stable bias current between start up and full temperature. With 200mA bias, I was able to get the bias fairly stable when the HS temp warmed slightly and for when it is hot, but the initial bias is a bit high. I don't think this is a problem if it is not too excessive, but others may have other opinions. I didn't dwell too much on the issue. I think Bob mentioned somewhere that he only placed one of the EC transistors on the heat sink and had better tracking results. The challenge is to get the EC transistors off the heat sink. My thoughts were to use a small signal SOT-23 device and mount it on the PCB inderneth the output transistor pins, so as it contacts the drain pin right were it enters the component. I've also contemplated using a daughter board for the EC amplifier.

As for the operation of the output stage, it has great potential to get real results from fairly cheap components.

Hi CBS240,

When you were having difficulty with stabilizing the bias, was that without either of the EC transistors on the heat sink?

There's a graph of bias vs time in the original JAES paper that compares bias stability with that of a BJT amplifier, and the MOSFET one looks quite good. However, different implementations may need different degrees or types of temperature compensation. I think I biased the original 50W amplifier at 150 mA with 35-volt main rails.

I recognize that it would be very nice to not have to have one of the EC transistors on the heat sink, since they and their part of the circuit want to be optimized for best high-frequency performance. In my original design, I think I used something like an MPSU07 for the EC transistor that went on the heat sink because it had a convenient mounting tab. Obviously, this is not the fastest transistor in the world and the need to have some wires to it doesn't help the HF either. I've often pondered ways to get the EC transistor off the heat sink, but have never bothered with it.

Cheers,
Bob

[CBS240]

Hi Bob

I have experimented with both transistors on the heatsink, then removing just one from the heatsink, to using a Vbe multiplier circuit in series with the EC amplifier. Even with one transistor on the heatsink, it would still track with a slight neg Tc, which is better than a pos Tc. Increasing the emitter resistors on the error amp helped the tracking by reducing the Tc of the error amp transistors, but it also reduces the gain of the error amplifier and that would lead to a pointless outcome. To me, in my 'home-made' amp, I don't really care much if it over compensates a bit, it doesn’t decrease that much so I just set the 'cold' bias a little high, but it’s not perfect.




Pondering…..If you mount the outputs horizontal to the PCB so that the transistor pins bend 90 degrees into the PCB, then left the drain lead a bit longer than the others so that under this pin you could mount a SOT-23 (EC amplifier) in contact with it using a solder pad that would allow it to 'slide' from the case of the output transistor to a few mm away so as to use the drain pin lead as a pot in the thermal model instead of a fixed resistor, assuming that it over-compensates when mounted closest to the output transistor. Of course you want to keep the leads as short as possible to impede parasitics, but it might be worth the trade off on the drain pin to be able to easily tune in the thermal bias stability by simply moving the position of a couple of transistors. The question is, could it be done neatly, with all the other needed circuit components on less than 3 layers.

[Bob Cordell]

Originally posted by CBS240
Hi Bob

I have experimented with both transistors on the heatsink, then removing just one from the heatsink, to using a Vbe multiplier circuit in series with the EC amplifier. Even with one transistor on the heatsink, it would still track with a slight neg Tc, which is better than a pos Tc. Increasing the emitter resistors on the error amp helped the tracking by reducing the Tc of the error amp transistors, but it also reduces the gain of the error amplifier and that would lead to a pointless outcome. To me, in my 'home-made' amp, I don't really care much if it over compensates a bit, it doesn’t decrease that much so I just set the 'cold' bias a little high, but it’s not perfect.




Pondering…..If you mount the outputs horizontal to the PCB so that the transistor pins bend 90 degrees into the PCB, then left the drain lead a bit longer than the others so that under this pin you could mount a SOT-23 (EC amplifier) in contact with it using a solder pad that would allow it to 'slide' from the case of the output transistor to a few mm away so as to use the drain pin lead as a pot in the thermal model instead of a fixed resistor, assuming that it over-compensates when mounted closest to the output transistor. Of course you want to keep the leads as short as possible to impede parasitics, but it might be worth the trade off on the drain pin to be able to easily tune in the thermal bias stability by simply moving the position of a couple of transistors. The question is, could it be done neatly, with all the other needed circuit components on less than 3 layers.

This is an interesting suggestion, although mechanically it would be a bit non-standard. Maybe what we need is some kind of opto-isolated thermal current mirror or conveyor :-).

I have not tried it, but putting some kind of a thermal temperature coefficient in the fixed main bias spreader (the 22V spreader on the VAS output) might also work, since there is some effect it has on the final spread. I just haven't evaluated how much that effect is.

Cheers,
Bob

[KLe]

Originally posted by myhrrhleine
for starters, we might consider the transistors used:
Q1, Q2 LS844 Linear Systems
Q3, 14, 15, 16, 22, 24 2N3904
Q4, 5, 19 2N5550
Q6, 7, 8, 9, 10, 11, 23, 25 2N3906
Q12, 13, 21, 27 KSA1381 Fairchild
Q17, 20, 26 KSC3503 Fairchild
Q18 2N5401
Q28 IRFP240
Q29 IRFP9240 Fairchild

Hi myhrrhleine

This might seem like a silly question, but where can I find the schematic? ...

[m2003br]

see the link at the first post...