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r_jik45 26th August 2006 12:38 PM

One more mini Zen... almost!
2 Attachment(s)
Hi all,

I'm finishing the construction of the mini-Zen headphone amplifier proposed by Tortello (diyaudio link, headwize link). Actually I followed a modified part list found on PT sound lab with Digi01's board in mind. Thanks a lot to them :up:

The amplifier will feed a Sennheiser HD600.

I'm now trying to set the bias current. According to Tortello's original description the bias current could be a bit lighter then 210mA for higher impedance headphones. I set it to 180mA.

Now I have some questions...
- Is 180mA a good bias current to feed the HD600 or is it possible to go even more down (the limit is 100mA)?
- I let the amplifier for about 30min without any input signal and without any charge connected to the output. The heatsink became quite hot :confused: while I expected it to be only warm if one consider that in other versions of this amplifier the transistors can be just attached to the box without any special heatsink. Are the transistors supposed to become really hot?

Thanks in advance for your advices :)


Babowana 26th August 2006 01:00 PM

Re: One more mini Zen... almost!

Originally posted by r_jik45

- Is 180mA a good bias current to feed the HD600 or is it possible to go even more down (the limit is 100mA)?

Norminal impedance of HD600 is 300 ohms.
So, I drive HD600 with rather high voltages (P = V^2/(2R)).
I do not need high current (P = I^2*R/2).

tortello 26th August 2006 06:40 PM

Yes, you can go down, but my advice is not to figure out a bias current lower than 100mA.
The original box base is about 220 x 280 x 2 mm. alluminium, and is more than enought to keep hexfet slightly warm.


r_jik45 26th August 2006 08:55 PM

Hi Babowana and Marcello,

Thanks for your replies.

Ok I will reduce the current, and check again for the heatsink. In case I still have the possibility to attach it to the box itself.

Have a nice end of weekend,


r_jik45 27th August 2006 09:20 AM

And... maybe better to ask now:

I'm not in the electronic field (but DIY is fine ;)).
Could somebody give me a simple explanation regarding what is this bias current and the good/bad points setting it high or low? :rolleyes:
(what I found is too much electronic specialist oriented so I did not really understand :xeye: )

Thanks in advance for that!!!

Babowana 27th August 2006 11:51 AM

I understand that the bias refers to a fixed dc voltage or a constant current source that sets the operating conditions for semiconductor devices. And, the bias refers not only to its amount, but also its direction--i.e. forward bias or reverse bias.

As you know, your amp gets life of Class-A by a Papa's special current source bias of Class-A that works as a constant current source on idling condition, but . . . wow! . . . automatically coverts into an active current source whenever it sees any ac signal. No more ac signal? Then, it comes back to the constant current source bias. It is an efficient way of biasing, which is efficiently boosting ac signal current into the output loads (speakers or headphones).

Many diyers claim that more indling current improves the sonic quality of amps. Meanwhile, I would like to say that the size of Vds is also affects the sonic quality of them. Therefore, I believe that the desirable sonic quality could be got by a reasonable compromise between the size of Vds and idling current.

r_jik45 9th September 2006 04:14 PM

2 Attachment(s)
Hi all,

Seems I need your help again...

I've lowered the bias current to 150mA.
I connected the potentiometer the wrong way; the volume decreases instead of decreasing when I turn the potentiometer from left to right. Well it's a minor problem for now.

I tried the amplifier with an headphone (balance not adjusted yet)... and realized that there is a big 'noise/hum'.
I made a scheme of the ground connections in order to see if there could be a problem there. The numbers refer to:

1. Ground right input (connector to pot.)
2. Ground left input (connector to pot.)
3. Ground right input (pot. to pcb)
4. Ground left input (pot. to pcb)
5. Ground resistance to case
6. Ground to mute outputs (relay)
7. Ground right output
8. Ground left output
9. Ground socket to filter
10. Ground socket to case
11. Ground (12V supply for relay)
12. Ground (12V supply for LED)
There is an aluminium sheet between the transformer/filter and the main pcb. It's connected to the case.

I cut the connection '4' between the potentiometer and the right input (pcb) but the hum is still there. Is it ok to keep the ground between the input connectors to the potentiometer or should I wire it directly to the ground-inputs on the pcb?
Another point is that the heatsink (where the transistors are connected to) is not connected to the ground. Should I connect it?

These are the things I'm thinking of...

What is your opinion regarding this wiring? Or what do you think I should do to stop this hum?

In case, the amplifier can be seen HERE .

Thanks in advance for your help... and have a nice end of weekend,

Babowana 10th September 2006 04:44 AM

Not only proper ground loop is essential . . .

But also locating input and ground wires clear of the transformer leakage flux is important. When I can't have clear distance, I "twist" the input and ground wires all together tightly.

Hope you will solve it soon . . .

r_jik45 17th September 2006 07:03 AM

Babowana, thanks for your reply.
Yes there are different possible sources of problem. But I thought that maybe an obvious mistake could be seen by 'experts' of the forum ;)

Have a nice end of weekend,

Babowana 17th September 2006 07:56 AM


Originally posted by r_jik45

But I thought that maybe an obvious mistake could be seen . . .

Understood . . . :D

From the picture showing your PCB board, I assume that PSU and AMP are there all together in one board. It means that AMP's grounding path might be mixed up with PSU's grounding path carrying all dirty electrons before cleaned up. By using a small knife or a sharp tool . . . I would cut off all PSU grounding path and separate it from the AMP's grounding path, and send the separated grounding wire of PSU directly to the star ground.

I hope this will help.

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