Increase power supply caps. Mosfet power amp.

martinz

martinz

Active Member
Hello,
I have a pro use power amp that I hooked to a preamp and have been pleased with the results for home listening. It's a 90's Acustica Export, model C300. I can't find info about it but seems a solid piece and the insides look nice and tidy. I don't know its wattage but have inferred from its power output transistors (2 couples of j50-k135 mosfets) that it must be around 75-100w/channel... I'm no expert at all and maybe somebody can correct me.

This amp has four 3300uf power supply filter caps but I feel that could be due to cost cutting. Would it be worth to increase them to say 4800uf or even more? Only in case no other changes were neccesary after increasing the caps
Thanks for any advice!
 
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I would opine that a designer building a professional amplifier around Hitachi lateral MOSFET's almost certainly designed a more than adequate power supply in all regards.

I would leave modifications to those confident they know better than the original designers.
 
3300uf is designed in for all the caps. If you're asking by increasing the gain on the amp by using higher uf caps there would be no effect on the output.

If the amplifier does not sound good, look at capacitors in audio paths.
 
martinz said:
Hello,
I have a pro use power amp that I hooked to a preamp and have been pleased with the results for home listening. It's a 90's Acustica Export, model C300. I can't find info about it but seems a solid piece and the insides look nice and tidy. I don't know its wattage but have inferred from its power output transistors (2 couples of j50-k135 mosfets) that it must be around 75-100w/channel... I'm no expert at all and maybe somebody can correct me.

This amp has four 3300uf power supply filter caps but I feel that could be due to cost cutting. Would it be worth to increase them to say 4800uf or even more? Only in case no other changes were neccesary after increasing the caps
Thanks for any advice!
Click to expand...


It isn't simply cost, A reason they do that has to do with the electrical characteristics of using four smaller caps.
Read this..
http://sound.westhost.com/power-supplies.htm#ripple
 
Thanks for the answers.
The amp sounds good, clear, defined IMHO, no problem with it!. I was not thinking in increasing gain or power output, I was only worried as usually i see bigger caps in amps of that power, i'm guessing about 80w/channel. FYi all caps are Rubicon
So if that's the way mosfet amps are i'll leave it alone. Thanks!
 
Those mosfet's circuits can run warmer. Keep the top open and don't stack on top of vents. Let it run in a while..
 
Binkman said:
Those mosfet's circuits can run warmer. Keep the top open and don't stack on top of vents. Let it run in a while..
Click to expand...

Good advice, I have two Hafler P3000 (150X2) MOSFET amp that runs warm like my old Yamaha Natural Sound Class A,AB Integrated A-1000 amps do. I have never stack on any amp and I use DC fans run from my USB port on my PC so no fan motor noise. The manual for my P3000 said to run the amp for about a hour before "Serious" listening:scratch2: I have always wonder why. YMMV on that one, jus saying what is in the owners manual.
 
It's a pro amp, more robust than most home amplifier designs, it is designed to dissipate more heat than it will likely ever generate.

Maybe the OP would grace this thread with a pic of his amp. Not much on the net regarding Acoustica.

Here's one of it's little brothers, the C201.

IMG_4266.jpg
 
bowtie427ss said:
It's a pro amp, more robust than most home amplifier designs, it is designed to dissipate more heat than it will likely ever generate.

Maybe the OP would grace this thread with a pic of his amp. Not much on the net regarding Acoustica.

Here's one of it's little brothers, the C201.

IMG_4266.jpg
Click to expand...

A brethern from the same family :) . Mine is put into a bigger box and with different layout and boards.
Do you know the power or specs of yours? I can only guess about mine.

Thanks for the advice about letting it breathe, right know I have the preamp on top.
I'd post pictures but don't know how to... I'll try to do it
 
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:-) adding capacitance in the Power supply is a long ..Proven.. method to increase an amps headroom and consequently seriously improving Bass and dynamics.
An amp circuit is only as good as the depth of it's power reserves.

One fellow has installed 88,000uf over the initial 8,000 with substantive result. I've tripled the cap supply (in Uf) on an amp with instant marked improvement.
Only those who haven't tried it for them selves raise objections.

Makers of mid/low priced units simply won't spend the coin OR the real estate to fit ideal Filter caps.
That amp in the photo has a massive Torroid and yet miniscule Filter supply caps.. odd imo.

But DIY'ers can fit whatever their wallets allow..
Do a bit of research/homework first! though.
Some knowledge is assumed.
Fitting bleeder resistors and bypass caps to the new array is required as well as finding space (inboard or even Outboard) to house the new found power.
Good luck :-)
 
Hi Bare,
I really don't know about this, but maybe increasing power supply caps in mosfet circuits is not the same as in other amps? What I understood from the answers above is that it is how it is by design, not for cost. I agree the caps seems small, but that is comparing with non mosfet amps.

Pictures as requested by Bowtie
 

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bare said:
:-) adding capacitance in the Power supply is a long ..Proven.. method to increase an amps headroom and consequently seriously improving Bass and dynamics.
An amp circuit is only as good as the depth of it's power reserves.
Click to expand...
I'm with you. The best sounding amplifiers in my experience have massive supplies, regardless of their output devices. I learned the value of increasing the capacitance in a couple of amps over the years myself. Anyone remember the Double Dyna 400?

Using 50V caps, I'm guessing the pictured amp runs 45V rails with 13,200 uF for a whopping total 14 joules. My 100 watt/channel '81 Threshold Stasis runs 65V rails with 50,000 uF for 105 joules, while the VTL MB450 tube amps have 250 joules each.
 
+1
IMO, the original 4 x 3,300µF power supply design is primarily due to cost considerations. From the posted interior picture, there is room on the power supply board to accommodate larger capacitors.

attachment.php


Increasing the total power supply capacitance will help avoid rail voltage sag under high current demands; this translates to better overall sound during musical climaxes. I suggest that the OP consider installing 4 x 6,800µF replacement capacitors (or even higher if there is sufficient space).
 
Thanks you all for the input.
Yes, there is space for the 6800uf caps. I have this nichicon KW couple for another project. They are a bit wider and about one third taller. I may get another couple and try then! Yesterday the right channel went into protection mode a few times, maybe it was due to poor ventilation? I still have the preamp on top of the power amp...
 

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martinz said:
Thanks you all for the input.
Yes, there is space for the 6800uf caps. I have this nichicon KW couple for another project. They are a bit wider and about one third taller. I may get another couple and try then! Yesterday the right channel went into protection mode a few times, maybe it was due to poor ventilation? I still have the preamp on top of the power amp...
Click to expand...

The P3000 has two 6800uf caps and with a flick of a switch it can go mono 400 watts. Move that pre/amp before you smoke that amp Bro. Purchase a fan :thmbsp:
 
I suggest that you verify the lead spacing on the pcb board before ordering replacement capacitors. I have used Nichicon KW capacitors previously, but mostly smaller ones in signal path applications; for power supplies, I typically use low ESR, 105°C types.

Mosfet output stages are often biased high, so good ventilation is a must; there a good chance that improving the ventilation will resolve your protection issue.
 
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