Beefed up Amp Camp Amp?

Metropolis

Member
2014-12-07 7:27 pm
Hi All,

I am a total novice - I have built an F5 (I had a lot of help), and Aleph J (also with much help) and the F6 twice. Additionally, I built a Bottlehead 2a3 SET and a handful of tube and SS phono stages. All were totally paint-by-number affairs. Suffice it to say that I can take directions and execute very well, but I don't really know what I am doing.

I have two ACAs that I am running as bridged monoblocks (v1.6 with 24v supplies). They sound great, however, I prefer the sound of the amp in the true single-ended stereo configuration. It is just short on power into my 12 ohm speakers (they are listed at 97db, but I don't know if they truly are that efficient). Of all the solid state amps I have owned, the ACA sounds closest to a tube single ended triode (and I don't have to deal with tubes). I think I just like the way single-ended amps sound.

This got me thinking: would it be possible to build an amp using the ACA boards (basically the circuit) but with more powerful output MOSFETs?

Before you say "dude, just build an Aleph J" - I have built one... I actually prefer the sound of the ACA, for whatever reason.

I think with some help from the knowledgable folks on this forum, I may be able to build a beefy ACA by using more powerful output MOSFETs.

Objectives:
  • Single box, stereo configuration
  • Roughly 10 watts of Single-ended output into my 12 ohm speakers
  • Use the existing ACA v1.6 boards without having to cut new traces or anything
  • Keep it simple
  • Safety, Safety, Safety

Assumptions:
  • I will need a larger chassis with greater heat sinking (probably get the deluxe 4u)
  • I will build linear power supply
  • I can use the same JFETs and ZTX450 transistors in the circuit, but most of the other values will need to be changed to account for the more powerful output MOSFETs

So, the first step is trying to figure out which output MOSFET to use. I pulled up the MOSFET the ACA kit ships with on Mouser - the IRFP240PBF. From there I filtered by the case size (TO-247AC-3), N channel, Gate Source threshold voltage = 2V and VGS = 10 (I picked these because that is what the IRFP240PBF is). I began looking for other MOSFETs that had datasheet specs that were similar to the IRFP240PBF, but had a higher power dissipation.

I came up with the following list:
  • IRFP264PBF
  • IRFP460BPBF
  • IRFP360PBF
  • IRFP360LCPBF

The IRFP264 has the highest forward transconductance (which, I think, is the signal gain) and the lowest Rds(on). However, it's Qg -gate charge is the highest of the bunch (I think lower is considered better in this application, but I could be wrong).

Conversely, both the IRFP460BPBF and the IRFP360LCPBF looked like the closest match to the IRFP240PBF on the spec sheets, with the obvious exception being the Power Dissipation is much higher.

Does anyone have any thoughts as to which MOSFET is a good candidate for this project?
 
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2 picoDumbs

Member
2013-09-06 9:35 am
Yeah no worries.
IRFP150
Or
IXTH64N10L2

You can also use IRFP240 if you want, no problem.

All of those can easily do the job.

I can help with the rest later, I'm out at the moment.

32V supply will get you in the ball park of where you want power wise.
 
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[email protected] demands .......

OK P=Urms^2/R

which means that you need 10.95Vrms ...... lets say 11Vrms for that

multiply with 2 then with 1.41 , and there is Vpp figure = 30.98Vpp

lets say 31Vpp

count on , say , 4V for voltage loss across devices (as minimum and back of napkin calculus) , so you need rails sum of 35V to be able to manage that power

similar logic is for calc. of Iq

at 12R as load , thinking about power as P=I^2 x R , we get I=0.91A

for standing current , we are using trusty thumb of rule ( we all have a thumb ?) that most funny A Class output stages are able to deliver twice Iq to load , so your Iq nees to be 0.91A/2 ....... and add some , to have any headroom

recalculating things around input JFet is wise........ necessary in fact , already covered somewhere in ACA thread

so , with 12R , you're not in bad position ...... 35Vx0.5A gives 17.5W of dissipation per channel ...... easypeasy

edit: while I was typing , Pico was lazy

:devily:
 
In order to get more power out of a stereo ACA with few modifications, you will need:
1. Higher transconductance Mosfets
2. Higher voltage and bias current

The known good choices for the Mosfets are the IRFP044, IRFP140, IRFP150, and my personal favorite, the FQH44N10. There are also some IXT parts that could work, but let's keep this simple. The highest power ACA that I've built so far uses a pair of IRFP140s with a 28V power supply. Look for my report in the ACA with Premium Parts thread. It's the ACA-220 v2. It sounds great, but is no match for my Aleph J or F6, both of which have special modifications of their own.

There have been reports of ACAs using IRFP150s with a 30V power supply. My experience with the FQH44N10 is with my hotrod F6. I would be tempted to run those with a 32V or 36V supply and 1.6A bias current. Bigger heatsinks will be necessary.

The input JFet will run safer if you can add an RC dropper above its drain connection. Try 499Ω or 1kΩ and 100 uF Organic polymer. The PSU should be a very low ripple design; set a goal for 28 to 30V and 10mV or less ripple. Again, check out the ACA thread that I mentioned for recommendations. If I were starting from scratch, I would use a 300VA transformer with 25V secondaries, LT4320 based synchronous rectification and a CRCRC filter.
 
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The unmodified ACA shows an increase in THD above 2KHz. I'm not sure if this is a flaw or a feature. It might make full range drivers sound more detailed. That little JFET has a tough time driving the Gate capacitance of Q1. (Anyone experimented with changing the Gate stopper of Q1 ?)

Anyway, if you want more high end THD, go with the 50 series HexFets. A 30 or even a 20 series HexFet will sound a little mellower. Perhaps one of the Fairchild parts will strike the right balance for you. See post 576 in the ACA sticky thread.
 

TonyTecson

diyAudio Moderator Emeritus
2003-05-29 2:57 am
Maybunga, Pasig City
i plan to upgrade the smps psu from 24vdc to 36vdc, will find out if the irfp240 can survive...

ultimate plan is 48dvc smps, i have a unit that can do 10 amps...looking for a real big heatsink...like the conrad engineering 35cm one....Conrad Heatsinks - Products

for the mosfets, i am using the ON semi FDL100N50F https://docs.rs-online.com/33b7/0900766b81538218.pdf

at 48 vdc B+ i doubt that the 2sk170 will survive...any jfet recommendations?

f i can not find a jfet with higher voltage specs, i plan on just adding a 24v zener in series with the drain...

i have about 70 channels of ACA build as a result of teach ins and build outs last year and i am hooked on the sound...
 
i plan to upgrade the smps psu from 24vdc to 36vdc, will find out if the irfp240 can survive...

ultimate plan is 48dvc smps, i have a unit that can do 10 amps...looking for a real big heatsink...like the conrad engineering 35cm one....Conrad Heatsinks - Products

for the mosfets, i am using the ON semi FDL100N50F https://docs.rs-online.com/33b7/0900766b81538218.pdf

at 48 vdc B+ i doubt that the 2sk170 will survive...any jfet recommendations?

f i can not find a jfet with higher voltage specs, i plan on just adding a 24v zener in series with the drain...

i have about 70 channels of ACA build as a result of teach ins and build outs last year and i am hooked on the sound...

use IRFP150

regarding 2SK170 - it's simple to change appropriate resistor to accomodate for higher voltage , already shown in ACA thread

I probably have that saved in my files , but not in this (workshop) PC
 

Sangram

Moderator
Paid Member
2002-09-25 11:01 am
India
You could simply cascode the Jfet when running higher voltages, will be 'better' than a zener.

The IRFP240 (each one) sees roughly half the voltage rail so will be fine up to about 60-80V without issues.

Output cap needs to be bigger. With bridged amps the cap is not required and I would stick a 30V supply on each amplifier, cascode the JFET, uprate all the cap voltages and call it a day. You will need to cut one trace on the amp board if you want to cascode but with a 30V supply there's no need.

I can agree that the sound of a single-ended amplifier is sublime. Though I haven't built an ACA, I did build a circuit with sliding bias and a SE output, designed with bipolar output transistors and a 2SK170 in common-source configuration at the input. It goes from 15V to 60V without any issues if you mind the component voltages. The only real limit is the gate-drain voltage of the 2SK170, which is about half the supply voltage in that circuit.

The Jfet in the ACA sees about the full supply voltage less the bias voltage of the output device.
 
Zen Mod, i already have those mosfets on hand...

would like to see the jfet mod to be able to run higher voltages..

i would appreciate if you can show links to the ACA thread..
i am a johnny come lately to mosfets...

point me to schm you want to use as basic one , I'll edit and post here

easier than find previous one
 
BIG ACA

anyway , here it is , sshot of one of earlier musings
 

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