8 channel green pre

Hi

This is my first post for this forum; I have been a quiet listener for a while.

I have been building 8 channel version of green pre amp referring to older schematic and building instructions that I found from the Kev's site.

I have build now all the pre amps and the PSU which was also presented on the same site. All channels are working now and only once I have had some smoke in the air... :grin:

The next phase of my project is to put all the stuff in the rack box. Before that I'd like to be sure about the PSU: Is the one I?m using enough for 8 channels or should I make some modifications? I'm using 55VA power transformer.

And another issue that has been on my mind, is that I would like to have un-balanced guitar input for two first channels. How should I do that, any suggestions?

Thanks in advance,

Ile

> The next phase of my project is to put all the stuff in the rack box. Before that I'd like to be sure about the PSU: Is the one I?m using enough for 8 channels or should I make some modifications? I'm using 55VA power transformer.

55VA AC should be good for 25 Watts raw DC, maybe 15 Watts regulated DC.

If I recall this Green, it eats about 20mA at +/-15V? 30V total times 0.020A is 0.6 Watts each channel. Even rounded up to 1 Watt, you have plenty of power.

> I would like to have un-balanced guitar input for two first channels. How should I do that, any suggestions?

Yeah. Don't even start with a mike amp. Low-impedance balanced mikes and instrument pickups are very different things. The quick workaround is to buy/build a couple of direct-boxes, to interface guitar to a lo-Z low-voltage balanced line that runs to the mike amp. There are plans floating around here somewhere. But unless the guitar is far from the preamps (guitars don't like long lines), converting down and back up is doing things the hard way. Also guitars often like more "color" than a clean mike amp will give.

Hmmmm... as long as you got Green boards, you could try this:


I think this can even be done without cutting the board, just omitting several parts, adding some jumpers. That could be a real brain-twist, not as easy as stuffing a board.

The point of taking out Q1 Q2 is that these were biased for low noise in low-Z souces (150Ω mikes). With a hi-Z source (guitar is 5K-100K) the Q1Q2 base current noise flowing through the hi-Z source will create much larger noise output than a mere TL072 alone. Q1Q2 would need radical re-biasing to work best with hi-Z sources. And the mild mannered TL072 is really quieter than a guitar pickup. So just use it (and swap some feedback to get things right again) (and use more guitar-level input coupling) (and omit the Phantom!). You should normally never crank that Gain switch all the way up with guitar pickup; you could use a 6-pole switch with resistors 68, 68, 150, 330, 820, 2K7, 150K.

And Welcome, Ile!

Hey PRR,
The TL072's are in there because Peters' simulator does not have a model for a 5532. They are actually 5532's.
Also, welcome ile :grin:
Regards,
Danielle

did you do a speacial board for that project? I `m building a 6 preamp , 3 pairs with diferent inputs chips: ina103, ina163 and ssm2019 .
I conect the input directly to the ssm2142 balanced autput. im not sure if I have the suficiente gain each chip could give 60db I think and the ssm2142 give 6 db more .

I think with more gain on the input less noise we have, I`m right? or I need to add a gain stage for better headroom?


I have the eagle file if someone want i, is not finish still.

There's also this possibility: a JFET+PNP circuit providing Hi-Z & cable driving and then on to the mic-input:

http://www.jensentransformers.com/as/as004.pdf

It provides a clean interface and will also work for mic-inputs without a TX.

Bye,

Peter

Very interesting PRR. Would you use the same value cap if going into a 5532?

Joel

[quote author="12afael"]I `m building a 6 preamp , 3 pairs with diferent inputs chips: ina103, ina163 and ssm2019 .
I conect the input directly to the ssm2142 balanced autput. im not sure if I have the suficiente gain each chip could give 60db I think and the ssm2142 give 6 db more . [/quote]

??
" I conect the input directly to the ssm2142 balanced autput. "
:shock:
what?

I'm not sure what you are up to but have you seen my 2017 project straight from the app notes ?
I also have a 4 times board.
see
http://www.recording.org/users/kev/mic_pre_general.htm

Kev, I'm in the middle of a 4-ch Green project (the new rev - 14, I think). How the hell do I power this thing?
is the Group DIY KDMP power supply (with it's own PCB) the right route or no?

i really don't know what's going on.
thanks

I like the SSLtech supply.

??
" I conect the input directly to the ssm2142 balanced autput. "
Shocked
what?

Click to expand...

sorry for the autput... =output

well not the input , the input stage.

http://www.recording.org/users/kev/Dual_Mic_Pre_BATT_2.PDF

why add a gains stage with the lm883???

the ssm2019 or ssm2017 can give you 60db of gain is not enough?

the rane preamp don`t have a gain stage.

[quote author="12afael"] ... why add a gains stage with the lm883???
the ssm2019 or ssm2017 can give you 60db of gain is not enough?
the rane preamp don`t have a gain stage.[/quote]

exactly
read the application notes of the 2017 and I assunme the 2019 and then read about the 2142. (Input imp of the 2142)
It also provides an unbal out
or if you like
it can be used for as an insert point for eq or comp etc

Hi, thanks for advices.

I meant that I add a switch on the front panel which changes the input source between a microphone and instrument.

Probably I should build a DI box between instrument input and pre amp card. I found a nice transformer for that usage, but the prices were a bit too high for my current budget. Transformer -> http://www.jensentransformers.com/datashts/dbe.pdf

Do you have any suggestions for a bit lower budget DI option for this case? It would be nice if it works with phantom power.

-Ile

Do you have any suggestions for a bit lower budget DI option for this case? It would be nice if it works with phantom power.

Click to expand...

If I understand your question correctly the answer might be the suggested stuff earlier in this thread:

There's also this possibility: a JFET+PNP circuit providing Hi-Z & cable driving and then on to the mic-input:

http://www.jensentransformers.com/as/as004.pdf

It provides a clean interface and will also work for mic-inputs without a TX.

Click to expand...

Bye,

Peter

Ok,

Sorry Peter, at the first point I didn't understand the schematic, but now I think I do. :?

Do you Peter or anyone have a ready PCB layout for this schematic?

Thank you, twice...

-Ile

OK, and let me know if there's something not clear. I must say though that for additional drive-capability I modified the buffer-circuit: (1) I'm not using the phantom-powering, (2) replaced the resistor connected to the PNP-collector with a current source.

Alternative idea (& a having a layout-suggestion right away) might be to copy the Hi-Z DI-input of Fabios '312-circuit. he uses a JFET & NPN. Not the same but alike. That one can't be phantom powered I think; the NPN is biased fairly hot.

Bye,

Peter

Do you Peter or anyone have a ready PCB layout for this schematic?

Click to expand...

It's just a few components, you could put it on perfboard if looks are of less concern.