AT-HA-5000 schematic

[EUVL]

To reduce the relay trigger level, reduce the 300R between the 2x 10k at the input of the LM393 in the protection circuit.
Best to set up a Spice file to test it out.

And yes, those UHC-MOSFETs has a special sonic signature.


Cheers,
Patrick

 

[Ricotjuh]

You could also calculate it. If you have defined the switching points for your own, you can calculate the values. But I do not expect it to be necessary. The engineers of Audio Technica will have determined these values critically. Considering the rest of the setup of this circuit / PCB.
But of course you are free to make adjustments.

 

[Ricotjuh]

You can halve the 300ohm resistor to 150ohm, then you will have setting values of about + 100mV and -100mV. In your case, 2 resistors of 75ohm

 

[blackdod]

Thanks guys.

Yesterday I got confused and left my amplifier "on" arround 24 hours and the initial values have almost not been modified, which means that everything works correctly. At roughly 6mV Offset on one channel and 3mV on the other. Mosfet temperature about 47º

Only an appreciation: Of course I don´t have more knowledge than ATH engineers but EMHO 150mV offset can fry an earphone and more if we talk about an ATH headset (low impedance and high sensitivity).

This is why I was surprised by such a high value and that is why I was asking if the safety circuit could be adjusted to lower values.

I don´t want to create any controversy or any misunderstanding.

 

[EUVL]

You will most likely be using an over-ear headphone with such a powerful amp.
So assuming worse case load of 30R.
0.2V DC equals to (0.2)^2/30 ~ 1.3mW.

Most morden day over-ear phones are specified at 200mW or so.



Cheers,
Patrick

 

[blackdod]

Yes, I use over-ear all time. An amplifier like that (or your F5ha ) does not make sense if you don´t use these headphones.

I know, today there are higher tolerances with respect to offset but it´s something that I don´t feel comfortable. Maybe I´m very paranoid in that sense

 

[EUVL]

Well, I did not say it still sounds good with 100mV.
I just said it will not burn your coil.

And our own SHPP is set at somewhere between 50mV and 100mV, just as reference.


Patrick

 

[blackdod]

Well
And our own SHPP is set at somewhere between 50mV and 100mV, just as reference.

Yes, I know and AMB e12 uses 70mV too. That's why I think that this could be a good place to cut and not 150mV.


Off topic: I´m working on your SHPP .

 

[blackdod]

Two week with my amp working without problems. Thanks to Ricotjuch for your support and help.

I have material for change the bias current (3K9 and 0,1R) but I still think which of the two modifications is the best option.

 

[Ricotjuh]

You're welcome. Just test and see what the best result offers. But if the amplifier is working fine now, why change?

 

[blackdod]

That is a good question. I also consider leaving it like this (Bias @136mA) but on the other hand I think it's far from it's working range

 

[EUVL]

Those UHC-Mosfets has very high transconductances and hence plenty of drive, even in Class AB.
And 130mA bias is way more than enough for any headphone other than electrostatics (vias transformer).

So if I were you I would not bother.
But then it is your amp, and the only way to find out is to try.

If your MOSFETs (in fact all active devices) came from those Chinese kits then I sugggest you change them with genuine NOS parts.
Those parts supplied with the kit are what they call refurbished parts.
They are removed from equipment (i.e. used, 2nd hand, ....) and they even have means to weld on new leads to make them look new.


Just my 2 cents,
Patrick

 

[Luke]

Hi Euvl, why change refurbished semis, semiconductors don't wear out do they?

 

[EUVL]

Depends on what they have seen (voltage / current), especially when they have been used in power amps.
Ref. Toshiba Reliability Handbook.


Patrick

 

[Luke]

OK, so maybe they have been stressed, still work, but not optimum condition. If that's the case it maybe better to buy the pcb. I don't think the mosfets will be easy to source.

 

[EUVL]

> so maybe they have been stressed, still work....

Question is when they might fail, and how.
I had failed MOSFETs before after one year at high bias.
So I would not use any unless I have to, and then only with headphone protection to detect any power device failure.

> I don't think the mosfets will be easy to source.

Correct.


Patrick

 

[blackdod]

Your opinions are always welcome Patrick.

About Mosfets: I understand that they don't "wear out". I guess that work and temperature extend or not their useful life until they fail.

I'm quite impressive with the sound of UHC mosfet. I've searched information about it but I haven't done much. Not even a category as such. What can indicate that a Mosfet is "UHC"?

 

[EUVL]

For MOSFET failure modes, please refer to the reference above.

UHC means ultra high conductance (or some say current).
These were made by Renesas a few years ago for high current switching.
Also used in many Japanese DIY amps (notably by Kaneda) as well as some products.
Plenty of drive and control, but perhaps not as "refined" as lateral FETs.

2SK2586 and 2SJ555 combo Mosfet
Hitachi mosfet power amplifier

2SJ554/2SK2955 classAB SEPP power amplifier
2SJ554/2SK2955 classA SEPP power amplifier
????????????????????(3) - ??????
https://www.minor-audio.com/bibou/amp/2006nonfb_pcb.html


Patrick

 

[EUVL]

For those intertested in circuits, here a Spice file of the schematics posted at the beginning of the thread.
There are no reliable models of 2SK2955/2SJ554, so I used the closest available, namely 2SK3497/2SJ618.

As you will notice, plenty of compensations all around required to get it stable.
Open loop bandwidth is very high, with about 40~50dB NFB.


Patrick
.

 

[blackdod]

In the end they have convinced me. I'll leave it like that and start working in its box.

There are some very interesting schemes and info Patrick but getting those Mosfet is not an easy task :-(