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Old 23rd October 2014, 08:16 PM   #1
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Default Replacing power MOSFETs in SDS Labs headphone amplifier

I need headphone amp and decided to go with schematics that uses OPAMP followed by push-pull transistors which provide current drive. Schematics that I found is SDS Labs Headphone Amplifier described on headwise.com (SDS Labs Headphone Amplifier | HeadWize). The schematics has PCB layout already developed for it. The AMB modified (AMB β18 Stereo Headphone Amplifier) it slightly like this:

Click the image to open in full size.

Original schematics is using Harris RFP15N05/RFP15P05 pair. The AMB version made with IRFZ24N/IRF9Z34N pair.

My idea is to replace output MOSFETs for IRF640/9640 or IRFP240/9240. The reason is because these are widely used MOSFETs that are available on eBay for moderate price.

Questions here are:
1. Will it be feasible to use such MOSFETs in the place of old ones?
2. If I use them, what changes, if any, should be made to the schematics?


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Old 23rd October 2014, 09:16 PM   #2
Nisbeth is offline Nisbeth  Denmark
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Why not just buy the IRFZs instead? You can even get matched pairs from AMB, so nothing further should be needed.

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Old 23rd October 2014, 11:08 PM   #3
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Sure, I can buy them from AMB, but those transistors are not as widely used as IRF640 or IRFP240 series. My goal is to use commonly used parts.

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Old 24th October 2014, 05:02 AM   #4
Nisbeth is offline Nisbeth  Denmark
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I am not sure I understand why that would be a goal in itself? As long as they are readily available and not marked as "end of life" then surely that's what counts?

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Old 24th October 2014, 05:31 AM   #5
ammel68 is offline ammel68  United States
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Quote:
Originally Posted by gooduser View Post
The schematics has PCB layout already developed for it.
Where is the PCB layout located? Are you referring to Gerber files, or what?
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Old 24th October 2014, 06:18 AM   #6
a1095us is offline a1095us  United States
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I have built the AMB 18 in it's original form. It's a quality headphone amp. His original parts are not that inaccessible. I don't get your obsession! Why this form?
There are plenty of opamp push pull designs out there that will perform in a similar fashion. Even the Elliot Sound Products version is practically indistinguishable from this using fairly common parts.
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Old 24th October 2014, 07:03 PM   #7
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Quote:
Originally Posted by ammel68 View Post
Where is the PCB layout located? Are you referring to Gerber files, or what?
It can be found on HeadWize website SDS Labs Headphone Amplifier | HeadWize
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Old 24th October 2014, 07:19 PM   #8
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Thanks everyone for reply.

I guess I have to change the original question here to:
What are the benefits of using IRFZ over other MOSFETs? Why Ti Kan, author of schematics, used them?

And for the sake of experimenting, I am thinking of trying IRFZ AND other MOSFETs. If anyone has worked with following transistors, can you share experience working with them?
Transistors: IRF640/9640, IRF610/9610, IRF510/9510?
Or even these: 2SK1058/2SJ162, IRFP340/9240?

Will the sound change if I switch them? Or I will not be able to hear the difference at all?

Thanks again.
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Old 24th October 2014, 07:26 PM   #9
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Quote:
Originally Posted by a1095us View Post
I have built the AMB 18 in it's original form. It's a quality headphone amp. His original parts are not that inaccessible. I don't get your obsession! Why this form?
There are plenty of opamp push pull designs out there that will perform in a similar fashion. Even the Elliot Sound Products version is practically indistinguishable from this using fairly common parts.
For CCS did you use CR470 diode, or substitute? I could not find that part in stores. I was thinking about replacing it with LM317? Like this:
Click the image to open in full size.

with 266 Ohm resistor for 4.7mA. Website - LM317 Current Calculator - Electric Circuit

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Old 24th October 2014, 07:35 PM   #10
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Quote:
Originally Posted by gooduser View Post
What are the benefits of using IRFZ over other MOSFETs
Lower capacitance.

The opamp has to deliver the current to charge/discharge the gates of the output devices.
The IRFZ duo is the best of the four you mentioned in post 1 (your IRF/P suggestions are horrible)
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