How to upgrade CDP head phone amp (NJM4580) - diyAudio
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Old 28th July 2004, 01:32 PM   #1
DIAR is offline DIAR  Finland
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Default How to upgrade CDP head phone amp (NJM4580)

The amp is very poor and I thought I could get much better sound by changing NJM4580 chip to a better chip. Any other suggestions?

I found out that AD8620 is a good choice but unfortunately very hard to find in Finland. Is there any good choises in RS or Farnell catalogues?

The Vss is 12 V regulated.

I use Grado SR-80 head phones.
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Old 28th July 2004, 03:01 PM   #2
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DIAR,

what is the impedance of your headphones?
NJM4580 should only drive into greater 1kohm loads.

I am asking that because NJM4580 is not
a bad part normally, though there are better ones.

A replacement could be a NE5532, which can drive into
600 ohm loads, if the lower impedance of your
headphone should be the problem. That's a very
cheap one and thus always worth a try. Many people
think NE5532 is a great OPAMP despite its age.

An OPA2228 is also a good replacement for NJM4580,
many users feel that it has very good sound,
combined with also very good specs. But
OPA2228 should be used only if the gain is greater
then 5, otherwise there can be instable behaviour.
For gains lower then 5, you might want to look at
the OPA2227 then. I am not shure about the
OPA2227 / OPA2228 capability to drive 600 Ohm loads,
from the datasheet you can assume that it will work.

I replaced NJM4580 against OPA2228 in a pro-ject
phonobox phonopreamp with good success, but
a phono preamp is another application than a headphone
amp.

greetz,
Stephan
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Old 28th July 2004, 03:28 PM   #3
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OPA2604 is OK for a headphone-amp. It sounds better than NJM4580 anyway ( though that one is less noisy ! ).
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Old 28th July 2004, 06:22 PM   #4
DIAR is offline DIAR  Finland
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Thanks for your answers!

The impedance of the head phones is only 32 ohms!

http://www.gradolabs.com/product_pages/sr80.htm

Both OPA2228 and OPA2604 would be a good choise because I can get it very easily and they costs only (?) about 5 euros. Should I choose OPA2604?

Is it ok to replace the original chip just like that or do I have to change some other parts also?
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Old 28th July 2004, 07:50 PM   #5
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For only 32 Ohms load I would always expect problems,
regardless what OpAmp you use. None of the "normal"
OpAmps is designed to drive this kind of load.
Find another Headphone, or get yourself an
external headphone amplifier. There is a lot of discussion
on headphone amplifiers here around.

Stephan
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Old 29th July 2004, 12:36 AM   #6
digi01 is offline digi01  China
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DIAR,
the AD823 is a good choice too.at the same time,you may do some modify for drive 32ohm load.remove the C225,C256 replace into a couple of 47ohm resistors.
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Old 29th July 2004, 06:05 PM   #7
DIAR is offline DIAR  Finland
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Is there a risk of DC in the output if I remove C255 and C256?

I tried to find out what kind of head phone amp is in my Sony STR DB1070 AV-receiver. It sounds very good, like tube amp. Very smooth but still very accurate. Allthough there is a hint of distortion in the sound sometimes. The amplifier seems to be some kind of Darlington transistor. Could this be right?
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Old 30th July 2004, 01:23 AM   #8
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Quote:
Originally posted by DIAR
Thanks for your answers!

The impedance of the head phones is only 32 ohms!

http://www.gradolabs.com/product_pages/sr80.htm

Both OPA2228 and OPA2604 would be a good choise because I can get it very easily and they costs only (?) about 5 euros. Should I choose OPA2604?

Is it ok to replace the original chip just like that or do I have to change some other parts also?
Have a good read about the meta42 and other headphone amps. It's a great resource. When a friend built a headamp for his grados, he found that the 32ohms was a major problem for just about every opamp. We then tried a buffer section like that on the meta42 and the clipping disappeared immediately. One option for you is to beef up the power supply to the 4580, then add a pair of buffers between the 4580 o/p and the headphone socket.
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