Replacement transistors/mosFETs for holton n-channel amp

[hulsebos]

Hello,

Currently I am trying to build the n-channel amp from Anthony Holton (see attachment). I ordered components, but some types were replaced by others because they were no longer available.

My question is: can the following replacements be done without problems/performance degradation?

IRF610 -> IRF630
2SC3289 -> 2SC4159

Thanks in advance.

[richie00boy]

I think you should be OK.

[ilimzn]

You might have a slight problem replacing the IRF610 in the phase splitter with a 630 due to increased gate capacitances of the IRF630. I doubt it would be anything serious, perhaps a somewhat higher THD at HF, as it's really only Cgd that counts here (as millet cap) because Cgs is largely compensated by the gm of the 630 being about as much higher as the Cgs compared to the 610.
I do have some reservations about some things in that amp. For starters, so the drain resistors of the bottom bank of MOSFETs actually do anything useful? Seems to me they should be in the sources, not drains. The 330 ohm series ressitor in the gate circuit of the bottom bank is also slightly odd, and the bias current source using a modified Maida regulator is bit on the complex side

[richie00boy]

I did wonder about the bottom MOSFETs and their resistors myself, it just doesn't add up for thermal stability as the stage is operating in a completely different way to the usual CFP trick where a faked p-channel is made. However, I think some people have built the amp and it didn't blow up?

[Cobra2]

Mine is in everyday use (24/7)

Arne K

[ilimzn]

Originally posted by richie00boy
I did wonder about the bottom MOSFETs and their resistors myself, it just doesn't add up for thermal stability as the stage is operating in a completely different way to the usual CFP trick where a faked p-channel is made. However, I think some people have built the amp and it didn't blow up?

I did not imply it was thermally unstable, just that the resistors in the drains of the bottom bak do not contribute any to thermal stability, what ever it is.

[hulsebos]

Originally posted by ilimzn
You might have a slight problem replacing the IRF610 in the phase splitter with a 630 due to increased gate capacitances of the IRF630. I doubt it would be anything serious, perhaps a somewhat higher THD at HF

Thanks! And about the 2SC3289 -> 2SC4159, is that allright?

As I read somewhere, the 0.33 resistors were not in the design initially, but they were added later to make sure the IRFP240 equally share the load/current.

[hulsebos]

Originally posted by Cobra2
Mine is in everyday use (24/7)

Arne K

Nicely build!
So, what is your experience with this amp?

[Cobra2]

It had a tough birth...It did not like +/- 75V and a tough load...
So I changed to dual 38v ac (+/- 60-65V) 650va trafos, 4 x 22000uF Rifa pr channel, and a THICK cooling-angle.
- All components are hand-matched...
It sounds very good, I have compared it to several amps, with new price up to over $ 8000 before it was surpassed...

That is performance that I'm happy with

Arne K

[ilimzn]

Originally posted by hulsebos
Thanks! And about the 2SC3289 -> 2SC4159, is that allright?

Shouldn't be a problem. In practise, BJTs are much more easily replaceable, the choice is much broader.

As I read somewhere, the 0.33 resistors were not in the design initially, but they were added later to make sure the IRFP240 equally share the load/current. [/B]

That is precisely my point, the lower bank will not share current properly, nor increase thermal stability with resistors in the drains!
Current sharing with MOSFETs is generally a problem, the gm is relatively low so for good sharing the source resistors whould be relatively high value (like 0.5 ohm or more) but that of course lowers efficiency and the effective gm of the device, the later admitedly linearizes it, but also increases output impedance (although in this particular case the infulence on putput impedance is not very problematic). A compromise solution is to use smaller source resistors but pre-screen MOSFETs for Vgs, so that differences between the ones used in one bank are not excessive. Without resistors, you would probably need a hundred or so to find groups of three sufficiently similar ones, with resistors, you can buy the exact number or maybe a pair or two more, and just group them for best fit - so certainly a cheaper solution for DIY.
That being said, just moving the resistors into the sources of the lower bank may have it's problems - the thermal bias compensation may end up overcompensating. The components around the IRF610 near the LM317 regulator may have to be adjusted some.