Replacement transistors/mosFETs for holton n-channel amp

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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.
 

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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 :)
 
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N-Channel

Mine is in everyday use (24/7)

Arne K
 

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richie00boy said:
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.
 
ilimzn said:
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.
 
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experience...

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 :cloud9:

Arne K
 
hulsebos said:
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.
 
Hi hulsebos

The schematic that you have is quite old, there is an update
But I can tell you what to change.

The 2SC3298 is best replaced with and IRF610 or 630 or 710 almost anything in that type of fet will work fine.
Replaced ZD4 with an 12volt zener from the 9.1 volt shown.

For Q13 try a IRF710 or something close to it.
Don'y increase the source resistors above 0.33 the amp will clipp assymetrically, From memory 0.47 will most likely work OK, But I would check it first.

The schematic you have shows the source resistors in the bottom bank at the drains, this works fine and it was something I was playing around with for a while, However put the resistors in the source legs, this will work much better.

Also the gate resistors values can be changed to 100 Ohms each

If you have any other questions just let me know.
 
The Saint said:

The schematic you have shows the source resistors in the bottom bank at the drains, this works fine and it was something I was playing around with for a while, However put the resistors in the source legs, this will work much better.


Hi Anthony,

would you mind tell us what you had in mind when "playing around with Drain resistors"? :D

Cheers Michael
 
The Saint said:
Hi hulsebos

The schematic that you have is quite old, there is an update
But I can tell you what to change.

The 2SC3298 is best replaced with and IRF610 or 630 or 710 almost anything in that type of fet will work fine.
Replaced ZD4 with an 12volt zener from the 9.1 volt shown.

For Q13 try a IRF710 or something close to it.
Don'y increase the source resistors above 0.33 the amp will clipp assymetrically, From memory 0.47 will most likely work OK, But I would check it first.

The schematic you have shows the source resistors in the bottom bank at the drains, this works fine and it was something I was playing around with for a while, However put the resistors in the source legs, this will work much better.

Also the gate resistors values can be changed to 100 Ohms each

If you have any other questions just let me know.

Thanks for providing me the changes to your design!
This is the first amp I will build.
I am currently reading Horowitz an Hilll, but I am not at the part on transistors/mosfets yet, so forgive me for asking "silly" questions.

So a few questions:

1. how should the IRF replace the 2SC3298? base -> gate, emitter -> drain, collector -> source?

2. is it ok to use 0.22 ohm instead of 0.33 (have loads of them), or is it better to buy new ones? (I will match the mosfets into groups)

3. what is the purpose of R20, can it just as well be left out?

4. are E1 and C1 neccesary? This reduces the gain at dc to unity if I understand it correctly. But isn't C3 allready sufficient for that purpose?

5. After applying all your suggestions and using the IRF710 at Q13, is the bias ok? or should some resistor values be changed? Can I still adjust the bias according to your the value in the manual?

Thanks!
 
Hi hulsebos

Sure no problem.

So a few questions:

1. how should the IRF replace the 2SC3298? base -> gate, emitter -> drain, collector -> source?

It goes Base to Gate, Emitter To Source and Collector to Drain

2. is it ok to use 0.22 ohm instead of 0.33 (have loads of them), or is it better to buy new ones? (I will match the mosfets into groups)

No you will need to use the 0.33 ohms resistor as the Thermal tracking has been optimised to these values

3. what is the purpose of R20, can it just as well be left out?

R20 can be left out, its original purpose was to reduce the gain of the bottom bank at higher frequencies so as the help with closed loop stability.

4. are E1 and C1 neccesary? This reduces the gain at dc to unity if I understand it correctly. But isn't C3 allready sufficient for that purpose?

E1 reduces the DC gain back to unity and allows the closed Loop gain to be set to a gain of 47.8. Without it the output offset voltage would be in the serveral volts and would drift.



5. After applying all your suggestions and using the IRF710 at Q13, is the bias ok? or should some resistor values be changed? Can I still adjust the bias according to your the value in the manual?

No changes are needed when using the IRF710
 
Ok thanx, answers are very clear! I managed to order some IRF610 finally, so hopefully I will be using them.

One last question:

Can all resistors (apart from 0.33 Ohm) be 1/4 watt, or do I need higher ratings at certain places? I can imagine R17 and R18 should be more than that, I didn't find it in the manual.


I managed to match my IRFP240's to a much higher accuracy than the +/_100mV you mentioned.
 
hulsebos said:
I managed to match my IRFP240's to a much higher accuracy than the +/_100mV you mentioned.

This is something I noticed myself lately, when you get MOSFETs from the same batch, the tolerances are not as high as they once were. 10 years ago, MOSFETs were notorious for giving you 3 different parts when you bought three MOSFETs with the same name :)
 
Re: Re..

ilimzn said:


This is something I noticed myself lately, when you get MOSFETs from the same batch, the tolerances are not as high as they once were. 10 years ago, MOSFETs were notorious for giving you 3 different parts when you bought three MOSFETs with the same name :)

That's good, within a group I managed less than 10 mv

Cobra2 said:
I used common metal-film 0,6W resistors all over, the 0,33 made out of 3 x 1ohm-2W (mounted underside).
Arne k

Do any of the resistors ever become warm/hot?
 
The n-channel Amp is finished and working (see attachement). I made a hard-wired version on a pertinax board. Apart from a hum problem the sound quality is very good: bass is very well controlled en midrange very very detailed and spatially accurate. Also the high midrange/high frequencies sounds very controlled, no rough edges, but to my taste slightly on the dark side. But allthough the amp hasn't been fully played in yet, I already noticed quite some improvements after an hour of listening.
My compliments to Anthony for the design.


I have a few final questions:

1. Does anyone have suggestions concerning my hum problems? I think I made a decent ground point, but I am not experienced in this field.

2. I'm having a not very slight DC at the output of the amp: around 100mV. What exactly is causing this? Can I remove it by placing a potmeter/adjusing a resistor value somewhere (for example in the preamp or phase splitter section)? And, if yes, would it be possible to remove the capacitors in the feedback part (C1 and E1) and still adjust the output DC voltage to zero?
 

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