Another new A75

[norb]

Hello everybody,

completed the electronics of my A75 a few days ago (i will build the chassis subsequently) and i am extremely happy with it already.

Many thanks to Mr. Pass and Mr. Thagard for an excellent project. No problems at all, the amp worked perfect and stable right from the beginning and surprised me by having no turn on/off-thump. Nice add-on.

My A75 is a dual mono with self-designed and made pcb´s. Changes are the TO-220 IRF630/9630 on the output stage, 0R82 source resistors and Q11 mounted on the output stage heatsink for I found that it improves bias current regulation.

The amp is set up unbalanced (have to build a preamp yet), 100% feedback, folded cascode with R21/22 = 100R, and sounds very clear, has a wide stage and a powerful and precise low end. My only direct comparisons are Onkyo A7 and H&K 655Vxi (sorry for that) and the difference is stunningly large. With any well recorded music, it is like wiping a (thick) layer of grey dust from a window you thought to be pretty translucent already. Speakers are Isophon Europa.

I will test some other setups over the next weeks and hear what happens.

One thing i tested so far is thermal coupling of the input-fets for better offset regulation, so far coupling of only the two N´s and P´s of the differential pairs has decreased offset regulation. I expect coupling of all input fets to yield better results, maybe somebody can share his experience on this topic.

Building this amplifier is a huge reward (also for your local electricity provider) and enjoyment. Thanks again.

Norbert

For the photo: lower pic shows one channel: top pcb is front end power supply, lower pcb is front end, blue wires are speaker out, the red and black wires with lugs go to the capacitor power rails mounted on top (upper pic showing complete channel in crude test setup), the black rectangular bits are the source resistors, the square ones the mosfets.

[MEGA-amp]

Looks good so far!! What voltages are you running on the front end and output stage?

[norb]

After warming up the front end voltage levels out at +/- 51V; output stage is +/-41 V with 120mA bias. Heatsinks are 45°C with fans at 800 rpm, so the Mosfets shoud feel comfortable.

I may switch to toroids with a few volts less, because the standard trafos do hum a bit and i will see how noisy they are once the chassis is completed. Luckily the fans are nearly silent.

[moe29]

I always think "serious amp" when i see A75 builds

nice job.

[jupiterjune]

completed the electronics of my A75 a few days ago (i will build the chassis subsequently) and i am extremely happy with it already

Well, that could be a mistake--

because you won't be able to listen to it while you are putting the electonics into the chasis. I had mine in an open unfinished chasis for over a year before I could bring myself to take it out of service.

Very nice work, BTW!

JJ

[Blues]

Originally posted by jupiterjune


Well, that could be a mistake--

because you won't be able to listen to it while you are putting the electonics into the chasis. I had mine in an open unfinished chasis for over a year before I could bring myself to take it out of service.

Very nice work, BTW!

JJ

Advice to the knowledgeable in electronics...build the chassis first. The sweet spot can be addicting!

[jacco vermeulen]

Originally posted by norb
TO-220 IRF630/9630

Cheapest MOSFET output devices around, pleased to see another 630/9630 A-75 constructor

the capacitor power

EPCOS B-41580 ?

[jupiterjune]

Jacco-

Cheapest MOSFET output devices around, pleased to see another 630/9630 A-75 constructor

I used 634's/9630's --does that count?

Is the metric equivalent of a #4 screw any easier to tap? tapping 48 #4 blind holes was an exercise in patience!!!!!

Norb--what input mosfet's did you use?
No experience here with thermally coupling them. Keep us posted as to how that works!

JJ

[jacco vermeulen]

Tapping regular UNC, Metric or oldy Witworth makes no difference, blind tapping always is a pita.
same goes for tapping fine wire , no matter whether it's UNF, Metric fine or BSF.

If they were cheap they count, i've yet to build a more affordable output stage than the A-75's.
I recall paying 20 times more for a comparable 75 watt Pd, TO-220 RCA RFP type MOSFET for another design.

[Vix]

Originally posted by norb
I may switch to toroids with a few volts less, because the standard trafos do hum a bit and i will see how noisy they are once the chassis is completed. Luckily the fans are nearly silent.

Oh, yes. They HUM. In any "normal" application this is not an issue, but when you listen at low volumes late at night this can be quite irritating. I have built pass amps (Zen V9 and F2) with standard EI transformers, and their HUM is the only flaw. I am also looking to replace them by torroid ones.

A-75 is a good choice, deserves a nice chasis