Behringer ADA8000 pre bypass questions

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ouch! 5-6 metres adat cable...

When you get your creamware system, be sure to get the Wordclock plate too! creamware don`t have WC in/out by default.

ITOH, I just tried the ada8k along the apogee da16 and I have no flanging probs here, with the ada either in slave or master mode. All WC synced (RME HDSP, DA16 and ADA8K)

Synthi
 
Great, so Behry made a nice little unit after all. :green:

I ordered the Creamware with Z link (firewire cables) which suppose to be even more jitter free as word clock if you use their A16 Ultra converters.
 
Hello everyone!

New to the forum ... I've learned SO much from you guys and have much appreciation for all who post here.

I don't have an EE background, but hopefully I can share some useful information.

****

So I made it a pet project to try an trick out my ada8000 after reading this thread and a few others on this forum...

Here's what I did:

I followed Joe's mod and sent the line input signal directly to the a/d chip by bypassing the whole input section, including the opamp. I replaced the two 22u electrolytics with some Black Gates and the 3n3 with 4n7 Wima poly per the al1101 data sheet's recommended value.

micprebypass-%20mod.JPG


On the output I replaced the stock opamp with a MC33079 per jacob's suggestion.

ada8000%20ouput%20-%20mod.JPG


Here are some sound samples I made which might help illustrate the differences w/ the mod and without.

I only modded one channel, so in this test, I ran a Beck tune in a loop back test through the modded converter and then a separate test through the stock converter. In order to magnify the differences I repeated this loop back test 32 times per file!

Although this is not something you would normally do in a mix situation, I think it's a good way to test how robust a converter is in it's ability to accurately reflect the source signal.

(FYI - all files in this test were clocked by the Apogee Big Ben external clock)

Here are the sound clips:

http://www.musicians-samples.com/Original Track.wav
http://www.musicians-samples.com/Mod x 32.wav
http://www.musicians-samples.com/Stock x 32.wav

I urge you to listen to them with good monitors or headphones... I'll let you interpret the results.

A couple things to note:

The ada8000 I have does not exactly match Jacob's schematic. A few revisions were made since Jacob drew out the original... the 3n3 capacitor between the + and - of the A/D inputs is now a 1n8, and a few resistor values were changed or just plain missing from the board (from what I could tell).

A couple questions:

1. The input on the modded channel is very hot, and adversely the output on the modded channel is very low. I want the input and output to reflect +18dBu = 0dbfs.

Can I just change the 220R resistors on the input (Joe's mod) to a higher value to knock back the gain? Can I change some of the resisters on the output to a lower value to increase the gain?

2. What exactly is the capacitor between + and - doing? The data sheet says "To remove high frequency noise at the differential inputs, the capacitor between the differential inputs should be located as close as possible to the input pins."

Is this capacitor acting as a lowpass? Can I use a lower value cap to allow in more high frequency information?

Thanks again for everyone's input! Experimenting with this mod was a lot of fun, and I got my first shot at working with SMD parts. (They hurt your eyes!)
 
If you put the 470 Ohm resistor back in between the plus and minus inputs of the A/D, by varying the ratio with the 220s you should be able to set the input gain. The three resistors form a voltage divider (or attenuator to be more accurate). Two 220s in series with a 470 should be a reduction of roughly 6db. On the output side, reducing the size of the 7K5s should increase the output gain. Right now the ratio actually causes a gain reduction (relative to the converter output) of .87. Don't change the resistors on the second amp. This amp needs to have a gain of -1.
 
[quote author="burdij"]If you put the 470 Ohm resistor back in between the plus and minus inputs of the A/D, by varying the ratio with the 220s you should be able to set the input gain. The three resistors form a voltage divider (or attenuator to be more accurate). Two 220s in series with a 470 should be a reduction of roughly 6db. On the output side, reducing the size of the 7K5s should increase the output gain. Right now the ratio actually causes a gain reduction (relative to the converter output) of .87. Don't change the resistors on the second amp. This amp needs to have a gain of -1.[/quote]

Great info!

So if I remember correctly... currently the modded converter is +10dBu = 0dbfs. So if I want +18dBu = 0dbfs... I need to pad the input gain by 8db.

If 470R = 6db reduction, then 470R + (1/3 x 470R) = 625R = 8db reduction?

Another question:

Would it be possible to to bypass the output opamp, just as the input opamp is bypassed?

Thanks! :guinness:
 
Hi Norman, Thanks for the infos. Could you please upload the original file again? Something is mixed-up and it sounds more aggressive than the others. :grin:
 
The right idea but in the wrong direction, I think. To increase the attenuation to 8db you would have to make the 470 ohm resistor smaller. 6db is a voltage ratio of .5 to 1. 8db is a voltage ratio of about .4 to 1 so the lower resistor in the voltage divider would need to be:

R2/440 + R2 = .4 Solving for R2 makes the resistor 293 Ohms (but you should check the math).

I believe you need to have some kind of output op amp. There is probably a better way to set the output stage up to drive a 600 Ohm line though. The delta-sigma converters need to have their differential outputs processed by a differential input configuration, but a look at the datasheet might explain it a bit better.
 
If you put two 220R resistors in series with the input and a 470R in parallel to form a U-pad, your input impedance will only be roughly 900 ohms. Not good for devices designed to work into 10k+.

I actually think the stock input stage doesn't look so bad apart from the TL chips, maybe. Keep in mind that the converters themselves aren't super high class. The input stage may be perfectly fine for what the converters are capable of. Upgrading the cap near the volume pot may do some good.
 
I actually think the stock input stage doesn't look so bad apart from the TL chips, maybe. Keep in mind that the converters themselves aren't super high class. The input stage may be perfectly fine for what the converters are capable of.
Click to expand...

I completely agree. I've found this circuit only to be sonically problematic at the highest gains - if you back off your gain some 30% from max. of pot travel, it is very decent indeed.

And unbeatable at the price.

Jakob E.
 
Okay, then upgrading that cap near the pot to a bigger value and perhaps a higher quality grade may be the way to go. I have an older Mic 2000 preamp that's constructed similarly, and using a bigger cap near the pot did improve the sound especially at high gain settings.

BTW. Did anyone spot the super low Rbb transistor? Toshiba 2SA1316 = 2 ohms Rbb. Not a bad part, actually.
 
[quote author="norman_nomad"]

I only modded one channel, so in this test, I ran a Beck tune in a loop back test through the modded converter and then a separate test through the stock converter. In order to magnify the differences I repeated this loop back test 32 times per file! [/quote]

Not sure if I understand this principle - the clips sound horrible to my ears, though the stock circuit seems better at handling the bass?
 
With this modification, your source would need to be able to drive 600 Ohms. Doing this mod implies, I think, that you have some kind of preamp ahead of this unit.

The problem with the original is that they use a relatively low quality op amp. You could sub something better in the original circuit such as a 5532 and improve the noise performance and drive capacity.

The unit I have is quite noisy, at the highest gain setting with the stock circuit, where I might want to run it for a ribbon mike, for example.

Regarding the A/D converter that is used, apparently RME uses this converter in some of their products. Granted, it will soon become obsolete since it doesn't support 96KHz or higher sample rates. Its other characteristics are comparable with (the now obsolete) differential input Crystal units.

As a platform for building a high quality analog front end for a digital recording system "on the cheap" this is definitely the way to go.
 
On a nother note, it seems that the differences between (cheap?) coverters lie mostly in the analogue parts. For instance the ADA8000 has 16dBu=0dBFS, the Alesis AI-3 (20bit) has 12.2 dBu in = 0dBFS=17.5 dBu out (their specs, seems strange, I know).
The RME ADI8 has 19dBu (high gain setting) and the Creamware A16 Ultra has 20dBu. The Digi002 has 18dBu.

Anyone read the Bob Katz article in Resolution, about your gear should be able to handle 26dBu without clipping?

Just for reference, Digital Audio Denmark goes to 27dBu (30 on input), I'm sure all high end ADC do.
 
I think you're confusing things here. You can configure the circuit around the converter chip to work at 9 dBu = 0 dBFS or at say, 19 dBu = 0 dBFS. Some soundcards and converter units actually allow you to select the 0dBFS reference level. But the actual dynamic range will remain the same. The Alesis/Wavefront chip, if I remember correctly, is speck'd at 104 dB dynamic range. Some did a loop through test with RMAA and got slightly above 100 dB range. So the stock ADA8000 circuit pretty much uses the full potential of the converter chip. If you want a better signal/noise ratio, better analog circuity alone won't do. You will need better converter chips such as AKM 5394 (about 120 dB). RME use various qualities of AKM chips, I don't think they ever used an alesis/wavefront chip.
 
Well, I may be wrong, but I don't know of any RME unit that uses Alesis/Wavefront converters, do you? I wouldn't be surprised if RME had uses Alesis/Wavefront ADAT chips, though.
 
Apparently one of the products was the RME AEB8-I Expansion board for boards that support ADAT interfaces.

Presonus also used these chips in some of their products.
 
[quote author="Michael Krusch"]Hi Norman, Thanks for the infos. Could you please upload the original file again? Something is mixed-up and it sounds more aggressive than the others. :grin:[/quote]

Very sorry about this!

Now the "original" file is the correct one.

Just a note - no processing was done to the original file other than to collapse it to mono to match the converter files.
 
[quote author="ask"][quote author="norman_nomad"]

I only modded one channel, so in this test, I ran a Beck tune in a loop back test through the modded converter and then a separate test through the stock converter. In order to magnify the differences I repeated this loop back test 32 times per file! [/quote]

Not sure if I understand this principle - the clips sound horrible to my ears, though the stock circuit seems better at handling the bass?[/quote]

The files have been updated....

What I did was loop the Beck tune from the converter's output back into the converter's input.

Then I took that looped file and ran it through the converters again...

Then I took THAT file and looped it through again...

32 times.

Just making copies of copies... in an effort to see how well the converter could maintain the same qualities of the source file.

I hope this make sense!
 
[quote author="Rossi"]... If you want a better signal/noise ratio, better analog circuity alone won't do. You will need better converter chips ...[/quote]

I'm aware of that. I was just referring to the ADC's limits in terms of level(power).
 

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