Transistors in input stage Behringer 1622

[RuudNL]

A friend of mine has a Behringer 1622 with a distorting input channel.
I suspect one of the input transistors.
They are marked "888" and "A06".
Any ideas about the full type numbers?  ( 2N..., 2SA..., 2SB... ? )

 

[JohnRoberts]

MPSA06 is a GP NPN transistor.

The 888 doesn't ring any bells...  BUT using google for "888 + PNP"  yields  2SV888 a low noise PNP from "cool audio".

I think there is some corporate connection between behringer and cool audio. So that is my strong guess.

JR

 

[gemini86]

looks the same as the ada8000 preamp. May help broaden your search.

 

[JohnRoberts]

That topology is very common, and no doubt borrowed, but obviously re-engineered to use Behringer's idea of value components.

JR

 

[gswan]

It's very similar to this one.
http://sound.westhost.com/project66.htm

I'd suggest that the 2n4403 and bc549 would work just as well, you need to change the pair in each leg of the diff amp to match.

 

[JohnRoberts]

Yup, they both probably copied it from the same place... That looks similar to an old soundcraft (?) front end, a lot older than that second link's 2008 copyright but there are numerous variations on that general concept. .

JR

 

[joaquins]

Is this a good aproach for a mix pre... I have an ADA8000 and I'm thinking in taking out the pres, but I don't have any other to use there in portable studio, I travel with my macbook, a m-audio and this to record 16 track, sometimes live, I use the preamps from the console I use in the place, but when I'm in a garage I need small pres... I didn't take them out because I'd like to have some pre and a direct input that doesn't uses the pre,

My question is if it is a good type of pre what should I do to make it work better? Making a recap would help or is there something else like improve the input transistors or other thing? the A06 are SMD so would try first changing the 1316 to BC560... Any tips?

JS

PD: the other option is to take away the whole input board and make some small pre to fit in there, maybe something like green pre but using as 3rd opamp the one that is already there, PS mod would be needed to drive 8 NE5532 too. Other option is the pre of the  Allen Heath GL2200 that uses just 2 transistors and half TL072 but to avoid changing the SMD feedback resistors I could take down all the values and with more current I'd get lower noise and no need to mod the PS.

 

[dryman]

How does the input distors? I had a similar problem with nearly the same preamp. The problem with mine are the gainpots. http://www.groupdiy.com/index.php?topic=50849.0

 

[tv]

I think this schematic is (on purpose?) drawn wrong.

(err. I think, therefore I are)

 

[RuudNL]

Thanks for all your suggestions!
I think the 'right' answer must be 2SV888 (nobody sells them??) and MPSA06.
I still have the low-noise 2SB737 transistors, so I hope this is the problem.
The distortion sounds like clipping. A signal injected at the insert comes out right, so the probem must be in the first stage.
2n4403 and bc549 might work, but I think noise will be much higher.

 

[RuudNL]

I think this schematic is (on purpose?) drawn wrong.

Personally I don't think so (anyway: I don't see an obvious error), but what could be wrong in your opinion?

 

[tv]

t9, t13 are connected wrong.

 

[RuudNL]

Mmmm... I still don't see your point.
The base of T9 is driven by the collector of T1, and the base of T13 is driven by the collector of T5.
That makes sense to me.
T9 and T13 are NPN transistors, so collector connected to a positive voltage, emitter connected to a negative voltage.
All Behringer microphone input circuits look more or less like this; I can't imagine that they are all wrong...

 

[tv]

In esp schemo linked above (project 66), input pairs are drawn correctly, in behringer, they are f..ed-up.

 

[volker]

Thanks for all your suggestions!
I think the 'right' answer must be 2SV888 (nobody sells them??) and MPSA06.
I still have the low-noise 2SB737 transistors, so I hope this is the problem.
The distortion sounds like clipping. A signal injected at the insert comes out right, so the probem must be in the first stage.
2n4403 and bc549 might work, but I think noise will be much higher.

Coolaudio IS Behringer, you can also directly order from them: http://www.coolaudio.com/products.html

 

[JohnRoberts]

I'd be inclined to use the same parts if possible. The current sources feeding the input pair current density looks like it is set for 1mA or so, perhaps a little light for 2sb737 (IIRC they like 2-3 mA for lowest noise voltage at 150-200 ohm source).

JR

PS: Yes, the current source emitters are cross connected, compared to the ESP variant, but the negative feedback of the opamp is holding those two opamp input nodes to the same voltage so it may be an intentional difference, done just to be different. I'd have to put both on the bench and look at how they handle square waves to pick a winner between the two. FWIW this is just a lower cost variant on using one opamp per input transistor to keep them operating constant current.  Twenty + years ago it may have been a cost savings that was hard to resist (2 transistors in place of 2 opamps, times the number of input channels).  Note: there is yet another variant of this that uses two opamps instead of three to accomplish the similar topology so not quite as expensive or real estate intensive... It's hard to imagine quibbling over a $0.20 opamp (or less) but multiply times a few tens of input channels it all adds up.   

 

[rocket512]

if you look at this datasheet (http://www.garrettaudio.com/DATA/2SV888_DATASHEET.pdf), you might conclude that 2sv888 = 2sa970

 

[RuudNL]

if you look at this datasheet (http://www.garrettaudio.com/DATA/2SV888_DATASHEET.pdf), you might conclude that 2sv888 = 2sa970

Yes, I noticed that also. If you compare the datasheets of the 2SA970 and the 2SV888, most of the specifications are the same.
The noise figures for the 2SV888 are a bit lower. It might be that the 2SV888 is just a 2SA970 selected for lowest noise.

 

[JohnRoberts]

if you look at this datasheet (http://www.garrettaudio.com/DATA/2SV888_DATASHEET.pdf), you might conclude that 2sv888 = 2sa970

Yes, I noticed that also. If you compare the datasheets of the 2SA970 and the 2SV888, most of the specifications are the same.
The noise figures for the 2SV888 are a bit lower. It might be that the 2SV888 is just a 2SA970 selected for lowest noise.

Or perhaps the old data sheet was more conservative. How many customers can actually measure such data sheet parameters with high resolution and accuracy?

Another possibility is a newer cleaner process could improve some specs. IIRC impurities in raw material increase 1/F noise in semiconductor junctions, and selected parts are parsing out worst offenders.

JR

 

[rocket512]

Have a look at the bottom of pg3 ;-)