Behringer SRC2496 A/D D/A with THS4032 - diyAudio
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Old 12th March 2007, 12:24 AM   #1
seeyou is offline seeyou  Greece
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Unhappy Behringer SRC2496 A/D D/A with THS4032

Hello to all!

First of all I am a newbie that requires all the help I could get!

I've been trying to mod my cheap Behringer SRC2496
http://www.behringer.com/SRC2496/index.cfm?lang=ENG

It is a very cheap A/D and D/A converter that goes up to 96K, cool device to experiment on.

Starting with the mod, I replaced the lousy chinese transformer with a proper toroidal.
Next was to change the lousy PSU caps. replaced 2x1000uF with 2x3300uF and 1x440uF with 1x1000uf, all Panasonic FC.

All great till here.

Next step was replacing 6x NJM4580 in the DAC/ADC stage and 2x in the phones and gain section. Searching a bit in this forum gave me the impression that a good replacement would be THS4032 for all of those 8x 4580s.

I replaced them and ended up with a kind of positive result. The unit is still working and the sound is SOOOOO OPEN, wide and punchy, I was shocked!!! But I have an overheating problem on all of those 8x THS4032. The opamps get so hot after 2mins that you can't touch them. Overvoltage should not the problem here because the device operates at +-15V. I guess that there is an oscillation going on because at high volumes I get lots of different weird ZZZZZ sounds that are very dependent even on the movement of my hand approaching the cirquit. Also the feeling of the pots are kind of strange, it's like they are faulty or something, I hear strange noises while rotating them. After that I swapped the old opamps back in, every negative aspect disappeared.

By doing a little measuring, I could draw the schematics around the opamps on paper. I could scan them and post them if needed.

Without having adequate knowledge, I guess that the problem starts at the decoupling of the voltage pins that are as follows:

Pins 4 and 8 > 100n > GND
Pins 4 and 8 > 22 Ohm >100uF > GND

So, is there something I should change? Should I post the schematics around the opamp that I drew? Please help me out!

Any help would be EXTREMELY appreciated! Thank you!
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Old 12th March 2007, 05:00 PM   #2
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Have you checked the data sheet for the THS4032 compared to the older NJM4580?

It says the OP-amp (THS4032) cant drive a capacitive load unless it is smaller than 20pF. Check all outputs (pin 1 and pin7) to see if there is a proper protection resistor for to avoid oscillations and unstable operations. If there is capacitors connected right on the outputs it could be necessary to apply a series resistor from the output (20-75 ohm maximum). But Im not sure about the solution due to I dont have any schematics for the SRC2496 to look at and you havnt verified the oscillation on a scope?


Due to the high bandwith, this circuit operates in class-A and because of that it will be running pretty hot (ca 0.5W heat dissipating for each IC) but could of course run to hot if oscillate.

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Old 12th March 2007, 05:25 PM   #3
Eva is offline Eva  Spain
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Oscillating op-amps distort the sound badly, and that's what you are enjoying and calling "SOOO OOOOOPEN", distorted sound. Most pre-amp mods are actually based in making some stage oscillate by accident, but don't say it too loud or some people may become upset... (Anyway, the kind of people that does this stuff neither have an oscilloscope to check nor care about stability at all).

The following text was taken from the first page of THS4032 datasheet (top left):

Quote:

FEATURES
· Ultra-low 1.6 nV/ÖHz Voltage Noise
· High Speed
– 100 MHz Bandwidth (G = 2 (-1), –3 dB)
– 100 V/us Slew Rate
· Stable in Gains of 2 (–1) or Greater
So now the question is:
Did you bother reading just the first few lines of the first page of the datasheet before replacing the op-amps?

And if your reply is yes, then, do you understand the meaning of the underlined text?

Also, did you ever take the effort to read through the datasheet to page 6 to find out that each half of this dual op-amp draws 7.5mA idle current???? Do you know that 15mA on +/-15V rails makes 0.45W idle dissipation???
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Old 12th March 2007, 09:11 PM   #4
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Default Re: Behringer SRC2496 A/D D/A with THS4032

Quote:
Originally posted by seeyou
[snip]Starting with the mod, I replaced the lousy chinese transformer with a proper toroidal.[snip]

Be aware that putting a toroid in a low-level system is a Bad Idea. Toroids are wideband transformers so any noise, interference and other junk on the mains flies right into the circuit. Rectifier caps are not very effective at higher freq noise.
Using an E-I core xformer is much better.

The reason that often toroids are used in power amps is because they have less losses and are cheaper for higher powers. But they are less good for noise free equipment.

Jan Didden
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Old 12th March 2007, 09:41 PM   #5
seeyou is offline seeyou  Greece
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Thank you both for your quick replies. Eva yes I read the datasheet and I noticed the underlined text. And no, I don't know what it means. Although I knew when I read it that it means something very important, I took the chance to do the mod as to play a little around, search for answers and learn a little out of it. As I said, I am just a newbie, a bobbist. That's why I need your knowledge. I learn fast.

I don't have an oscilloscope to check the stability. Only thing I can do is to experiment and listen. Maybe I will throw the Behringer away if I destroy it, but till then, I do have some time to experiment.


Flodstroem thanks a lot for your hint. In deed there are components at pins 1 and 7. Actually there is a cap and a resistor in parallel (20p & 30K), going from pin 1 to pin 2 and from pin7 to pin 6.

I kept experimenting a little longer. I connected extra caps 220uF from 4 to ground and from 8 to ground (correct polarity) and the buzzing and the heating was reduced a lot, but is still there. Should I keep them?


All components connected directly to the opamp are:

Pin 3 > 30K resistor
Pin 5 > 30K resistor
Pin 4 > 100n > GND
Pin 4 > 22 Ohm >100uF > GND
Pin 8 > 100n > GND
Pin 8 > 22 Ohm >100uF > GND

Pin 1 > 20p > Pin 2
Pin 1 > 30K > Pin 2
Pin 7 > 20p > Pin 6
Pin 7 > 30K > Pin 6



Any ideas?


if it is useful I will track what is connected afterwards those components so I can provide a clearer picture.
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Old 12th March 2007, 09:42 PM   #6
seeyou is offline seeyou  Greece
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Hello...
I just found out via mesuring that the schematics aren't the same for all 8 opamps... in fact there is a different one for every opamp.... So I guess that the THS4032 is a bad choice for what I'm doing due to being so sensitive regarding oscillation.

Is there at least another good sounding opamp that is better sounding than the 4580, that can be implemented more easily than the 4032 without being so sensitive and that also sounds great? would the 8066 be a good candidate?

Thanks in advance!
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Old 12th March 2007, 09:42 PM   #7
seeyou is offline seeyou  Greece
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Ok, I will keep trying a bit more before throwing away the 4032s. Thanks for the Toroid information . I always thought that toroids where better than this.


Regarding the cirquit, I already mentioned that with more decoupling caps on the power rails, most of the buzz disappears and the opamp is getting cooler.


Flodstroem, regarding the resistor, maybe I will have to do some research. In one case there is a 100p cap going from pin1 to pin2. Where would I need to put the resistor? between lets say pin 1 and the cap? I will experiment with all the info I can get from you and I could post you the results. Just tell me what to do! I know that maybe it's not going to work, but it's alright!

regards and thank you!
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Old 12th March 2007, 10:08 PM   #8
Eva is offline Eva  Spain
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You can't use the THS4032 for most audio applications because IT'S NOT UNITY-GAIN STABLE, and most audio circuits require that condition. This op-amp is only stable for gains of 2 or higher, as it's intended for video buffering applications. There is no simple way to make it unity-gain stable.

Also, you can't use the THS4032 with +/-15V in a PCB designed for conventional op-amps because it requires specific layout considerations in order to handle those 0.45W of idle dissipation. Furthermore, most PCB layouts designed for conventional audio op-amps just can't handle the 100Mhz bandwidth.

Your choice was completely wrong... Consider reverting everything to it's original state if you want it to work properly.
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Old 13th March 2007, 04:46 PM   #9
seeyou is offline seeyou  Greece
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Ok Eva, I will. But I don't like the sound of the Behringer, that's why I started this whole thing. The device is used for audio mastering purposes and my budget is very tight. If you have the knowledge, what would YOU suggest to do? which opamp would you pick? it should be safe to say that AD8066 is out of the question too since it has high bandwidth. But I have read somewhere that the 8066 is used for replacing the behringers original opamps. Please help me a bit. Don't make me leave this forum
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Old 13th March 2007, 05:20 PM   #10
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seeyou,
first thing you should do if wanting to mod circuits should be to purchase a decent oscilloscope for a couple of bucks. Analog instrument are nearly for free now a days and that will be your first choice measuring instrument to use if your intention is to modify circuits. Also, you will learn a lot when using such an instrument and also, its very very useful for all electronic developments, error measurements, modifications and services on broken/faulty equipments.
Otherwise all your future works with modifications will newer be verified only estimated by a guessing.

It will do with a scope in the range of 50 to 100 MHz, (best=100MHz) but you dont need a 2 channel scope, but its very convenient to be able to see what is coming in and what is coming out at the same time when measuring.

Besides that, I only can recommend you to listen to Eva´s and janneman´s suggestions carefully.

Regards
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