Help On Op Amp Options

[richloh]

Hi,

I am trying to improve the sound quality from the Alpine TUA-T100DAB tuner as this is the only non-digital source into my H900 processor and started trying out different op amp to substitute the NJM4580s seen. With AD8599 replacing all the NJM4580s, I get good bass but sound quality became aggressive and harsh especially for the vocal ... not sure if op amp needs burn-in period but it is not oscillating as scoped.

I checked the service manual again per enclosed and note that there are LPF stage, 2 buffer stages and this ISO amp which I am still trying to find out learnt about them.

Can anyone help to advise on the following:
- why is there 2 buffer stages as I did not see 2 buffer stage in the block diagram ?
- ISO amp would not affect sound quality and hence can be left alone ?
- any recommendation for the Op Amp replaced for the LPF stage IC5, Buffer Stage IC8 and Buffer Stage IC307 (or even ISO amp IC308) ??

Please also advise what are the consideration for choosing "better" op amps for the mentioned LPF stage, 2 buffer stages and ISO amp, if at all possible.

Thanks in advance.

 

[Leon08]

NJM4580 are good op-amps.Just leave them in place.You can't improve much there.You can put a better op-amp, but you will not notice this because the sound depends much more on other things, such as loudspeakers for instance, much more.

 

[phaselag]

From what I can see on the schematic, there are 2 op-amps, both configured as followers (unity gain buffers). Both are fed from the same source and feed two separate "nodes" which seem to feed multiple circuitry. The buffer configuration is typically used to provide a low impedance source to feed multiple loads.

paul

 

[richloh]

Hi Leon, Thanks for your advice on keeping the NJM4580s in my DAB tuner on analog output only but there was indeed notable difference with the AD8599 but just that it was too aggressive and change the midrange to distance and unnatural sounding. I have already very good handmade Micro-P Z-series 3-way speaker, the Alpine F1 H900 processor etc for a high-end car audio system. My other digital sources of CD and DVD Changer are fed to a M-Clock Plus for upsampling, oversampling as well as jitter reduction before going into the F1 H900 processor. Regards,

Hi Phaselag, thanks for the information. Alpine technical person came back with this: "As you say IC5 is the filter IC307 and 308 are buffer/iso one for each Ainet bus socket as you can plug the cd changer in and switch between the dab and the cd changer" and I am quite sure now that IC307 and IC308 are not SQ related. Nevertheless, any recommendation for good LPF + buffer Op amp (instead of the twin NJM4580) with the WM8716 DAC installed is what I hope for. BTW, the voltage supply to these Op amps is 8.2V from the complete service manual. Regards,

 

[Leon08]

I just read the datasheet and don't understand why it does not work by you.

​

"The AD8599 is a dual, very low noise, low distortion opera-tional amplifier ideal for use as a preamplifier. The low noise of 1 nV/√Hz and low harmonic distortion of −105 dB (or better) at audio bandwidths give the AD8599 the wide dynamic range necessary for preamps in audio, medical, and instrumentation applications. The AD8599’s excellent slew rate of 16 V/μs and 10 MHz gain bandwidth make it highly suitable for medical applications. The low distortion and settling time of the AD8599 make it ideal for buffering of high resolution data converters. "

You must make something wrong.

 

[richloh]

Hi Leon08,

Thanks again and indeed AD8599 was recommended as upgrade replacement for NJM4580 but it is probably on the I/V stage which there isn't any in my case. These NJM4580s are for the LPF stage and buffer stage and while someone did also recommended AD8599 for buffer stage, there was very little being mentioned on what are good op amps for LPF and buffer stage which I am not sure if it differs from I/V.

Someone did mentioned that AD8599 has integrated buffer stage and to bypass the buffer with dummy or simply shorting the pins of the buffer stage .... not sure if this is my issue of having AD8599 in both LPF and buffer stage.

In summary, what will be good replacement for NJM4580 in LFP and buffer stage if AD8599s with integrated buffer in these stages do not work well ??

Regards,

Richard

 

[Leon08]

You should not believe anything what other talk to you.Some people don't know what they are talking about and only complicate things.For audio use you should look at specs in the datasheet.The most important are noise, distortion, bandwith and slew rate.All other is not important for audio.And if you look at AD8599 specs then you will see that it's very good suitable for audio use.So if you experience some problems with this op-amp then maybe because it's a fake or something else is wrong with it, but otherwise I cannot imagine why it should sound bad.

 

[Bonsai]

I also took a look at the AD8599 data sheet and there is nothing in there that would indicate that it should not do a good job. However, decoupling, layout, load impeadance etc all play a part in op-amp performance, so I agree with Leon08's earlier statement. You really need to be able to look at the output signal on a scope, or even better still with a distortion analyzer, to be able to judge whether changing one op-amp for another gives measurable improvements (not, I used the phrase 'measurable improvements'). Having said that, why do want to change from the original op-amp to the AD one?

 

[richloh]

Hi Leon08, I bought the AD8599 from Farnell Singapore and hence doubt they are fakes. Based on the specification, should it be good for LPF, Buffer and ISO amp or perhaps just use it for LPF instead?? Yup, be try that out and will also do some measurement with the scope to try understand what is going on ....

Hi Bonsai,
---------
It is said so often that NJM4580 are jelly bean stuffs and indeed the AD8599 specification looks superior in most if not all aspect and looking for general improvement of lower noise and/or overall better SQ ?? Again, should it matter on SQ for change in buffer and iso amp or just the LPF ?? Thanks again.

Richard

 

[Tekko]

LM4562 is the one choice if your after good sound with ultra low noise and distortion.

 

[Leon08]

If you want to use an op-amp as buffer it must be unity gain stable. In the datasheet of AD8599 is mentioned that it is unity gain stable.Means, should be no problem to use it as buffer.