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Old 21st February 2013, 06:45 AM  
owdeo is offline owdeo  Australia
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Originally Posted by abraxalito View Post
RC passive filtering only gives 20dB/decade filtering. Say typically there's a pole at 200kHz to maintain 0.1dB flatness at 20kHz. Then your rejection is only 40dB by 20MHz. Many DACs nowadays have OOB noise in the 100kHz to 3MHz range which barely gets attenuated under such schemes. But IME the main noise issues concern the noise on grounds - then the RC filter is actually making matters worse by providing a low impedance path for ground-borne noise to enter the opamp's input.
Good points thanks - food for thought. So if this really is the main cause of disappointing SQ from many opamps that look good on paper, you could say we're stuck with needing to use RF resistant opamps (or discretes) as there's no convenient way to prevent these kinds of noise getting in completely. Perhaps use of additional LC filters in the power supply would help with mains noise, and using balanced interconnections should sort out noise picked up on cable shields, but what can you do about DAC OOB noise...
Old 21st February 2013, 07:05 AM  
owdeo is offline owdeo  Australia
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Originally Posted by Bonsai View Post
I've built two preamps using this device, and it's single equivalent. I've measured the one design on an AP and the results are very good. I also built (on Veroboard) an experimental class A buffer using the 4562 as the driver to a discrete output stage and measured that on an AP. I listen to the preamps regularly and they are very open with great sound staging. So when people tell me so and so opamp is no good or I sounds bad I always wonder about their implementation.

I have a 200 MHz band width analog cope and a 1GHz DSO. You'd be amazed at the things you see that just don't show up with a 20MHz scope, or in some cases, builds where the individual has no access to decent equipment.
So did you measure any differences between the same circuit using the dual and the single versions? What were you measuring?

I have a digital 200MHz scope and an analogue 50MHz scope. Normally I can see what I want to see more quickly and easily on the analogue scope - eg ringing on square waves and signs of instability. Though the averaging can be handy on the digital scope. But rarely has the 200MHz revealed anything that wasn't visible on the 50MHz. What sort of things were you getting at?

Have you compared the SQ of your designs to others, perhaps with some unbiased fellow listeners and in a blind fashion? Are you experienced at listening to acoustic concerts and know what natural instruments should sound like? Sorry to ask, but I think these are reasonable questions given you're telling us how great your own designs sound...

My experience has been that the LM4562 sounds artificial, it is tonally "hard" and unpleasant to listen to. I found I wanted to switch the system off and stop listening. This is normally a good sign that something is wrong! I was unable to see any signs of instability in the circuits I tried it in, but who knows what else could have caused it. But since the two circuits were completely different designs I tend to think it's down to the opamp itself.
Old 21st February 2013, 10:18 AM  
Bonsai is offline Bonsai  Europe
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I am not making any comparative claims about how 'great' my designs sound over anyone else's designs - I am quite sure there are many other great designs out there. I simply stated that the 4562 sounds good and yes I have listened to a lot of equipment over the years so I think I am qualified to comment.

You ask if my stuff has been listened to by someone that is unbiased. I might ask you the same question about the sound of your construction.

I recently had two cases where I had HF oscillation up at over 100MHz that would have been visible with a low bandwidth scope. 50MHz is not bad, and clearly your digital scope would find a lot of this type of stuff as well.
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Old 21st February 2013, 11:03 AM  
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Originally Posted by owdeo View Post
... what can you do about DAC OOB noise...
That's precisely what my latest blog entry is designed to ameliorate :
The heart ... first dictates the conclusion, then commands the head to provide the reasoning that will defend it. Anthony de Mello
Old 17th March 2013, 06:41 PM  
Mooly is offline Mooly  United Kingdom
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Originally Posted by DouglasSelf View Post

You have to understand how Elektor works. They chose some of the parts, without any consultation with me. My prototype used parts from Omeg, which were wholly satisfactory and much cheaper.
I know its a while back, post #213

Would those pots you mention be the Omeg ECO types as here, or one of the other (metal enclosed) types Omeg produce ?

Potentiometers. Cermet vs conductive plastic vs cost and other questions.
Old 18th March 2013, 05:19 AM  
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I bet that discrete output gave open caracter to 4562 in your circuit. Opamp itself is indeed dry sounding, but that is how extremely linear opamps sound.
Old 18th March 2013, 05:32 AM  
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Some people use expensive Burr Brown current booster BUF634 (slew rate = 3000V/uS) at opamp output. It is very practical because both N an P transistors are in one case. But I heard that it tends to give softness and worm character to sound, which is not what I like.
Old 21st March 2013, 10:06 PM  
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I've "studied" Doug Self's preamp and I enjoied it. It is quite interesting and there are a lot of "tricks".

There is one of them I'm not shure I have fully understood the way it works.

I'd like if Mr Self could confim (or correct) my guess

I'm not sure I can publish the schematic on this forum so I'll refer to the schematic published on Elektor.

The circuit I refer to is named "load synthesizer"

Self writes:

"A load-synthesis circuit around IC4 is used to make an electronic version of the required 47 KOhm resistor from the 1 MOhm resistor R16. The Johnson noise of this resistor is however not emulated"

with no further details of the circuit.

An analysis by inspection of the circuits tells:

RL= R16 / (1 - A)


RL= is the "synthesized" resistor
A= is the gain of IC4A (the gain is inverting so A=~ -16)

Vn= en / A (on the first order and neglecting OP-Amp voltage and current noise)


Vn= is the noise of the synthesized resistor

en= is the Johnson noise of R16

A= as above


- en rises as the square root of the R16 resistor value

- A rises linearly whith the rise of the R16 resistor value

hence the Vn noise decreases as R16 increases.

In our case we have that the ratio of the noise of a 47 KOhm "real" resistor and the "synthesized" one is SQRT(16)= 4 ie the noise is 4 times lower.

Is the math correct?
Any further clues?

Old 25th March 2013, 07:30 AM  
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Originally Posted by diy_audio_fo View Post
Vn= en / A (on the first order and neglecting OP-Amp voltage and current noise)


Vn= is the noise of the synthesized resistor

en= is the Johnson noise of R16

A= as above
The above formula con be simplified ad follows:

Vn is "proportional" to en / sqrt (R16)

Last edited by diy_audio_fo; 25th March 2013 at 07:36 AM.
Old 12th April 2013, 06:52 PM  
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Default New Doug Self Tone Control

As promised earlier in this thread Jan Didden has published Doug's "A low-noise preamplifier with variable-frequency tone control" in his Linear Audio magazine. My copy will soon be in my mail box!

See details here: Linear Audio magazine
Best Regards,
Carl Huff


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