Grommes 6 Channel Mixer Into Individual Mic Pres

[tablebeast]

I have a new project brewing and I was hoping to get some input from you knowledgeable fellows before I dug too deep. I bought this Grommes 6 into 1 mixer (from the 80's I'm guessing). I think they still make it actually. Anyway, so the important thing is the chassis is super nice, it looks great and it has input transformers already that sound damn good. I want to replace all the audio electronics (which are lame), but keep all the passive parts (jacks, transformers, switches, chassis, knobs, pots) and power supply. I got a sample package from THAT with 4x 1510 and 4x 1512 ICs. I'd like to have 6 mic channels and a single stereo DI that will replace the master section. So that is where I am. I have the parts and idea, I just need to get a good pre-design and I'll be all set.

So, looking at the application notes for the 1510/12 I notice that it seems to be super-simple, even too simple. I'd like to make a basic circuit and since I am using an input transformer don't I get to leave out a lot of stuff on the input? I see lots of 1510 mic pre circuits out there but I haven't seen any with transformers. Anyone know of a schematic for one they could recommend?

Also, how do I feed an unbalanced signal to the 1512? The DI will be fed Hi-Z instrument or RIAA filtered turntable signals. Do I just use a coupling cap and ground the '-' input? If a cap, what size?

For the output on the 1510/12 do I need a coupling cap if going to an unbalanced output? Again, if so, what size?

Here is the datasheet for the THAT 1510 / 1512: http://www.thatcorp.com/datashts/1510data.pdf

Thanks in advance. I will get some pictures up soon.

Jesse

 

[tablebeast]

Anyone? Good idea? Bad Idea? Get lost? Two days and no interest. Am I just not getting something here? Is the THAT 1510/12 not suitable for my needs? Any help out there?

 

[PRR]

Everybody is waiting for the pictures you promised us.

Grommes stuff usually wasn't "lame".

The 1510 is a transformerless mike input. Why would you use it with transformers?

Do we have a clue what the transformer specs are? They are probably step-up and maybe high ratio... that puts you WAY off the impedance zone that the 1510 worked so hard to optimize for. And since you don't need no differential input neither, you could think about 5534 or even FET-input opamps, depending on the ratio.

I'm not sure how you know the transformers "sound damn good". Didn't you have them hooked to the Grommes' preamp? Maybe the preamp sounds good, or the tranny and preamp are synergistic.

 

[tablebeast]

PRR, thank you for replying kind sir! This is what I was getting at. It seems like I am way off trying to use the 1510, so steer me in the right direction and I will rename this thread.

I pulled one of the transformers from the unit and checked it out in a separate circuit and really liked the sound. In the Grommes it doesn't sound nearly as good, so I assume it is the Grommes Electronics that are sucking. I'm not familiar with the opamp in channels 1-4. It is an RC4156 and here is it's data sheet: http://www.datasheetcatalog.org/datasheet/fairchild/RC4156.pdf

It doesn't look too terrible on paper, but I would like to improve it still. I'd prefer separate ICs for each channel instead of one big one for 4 channels. Plus I don't need the mixing, EQ, or paging stuff so I figured I would just gut it and start over with the preamp circuits.

I don't have pictures yet, but here is a link to the data sheet for the Grommes G222 Mixer:
http://www.grommesprecision.com/precisionelectronics/assets/pdfspecsheets/g222.pdf

I think the input transformer is 600:10k so what kind of circuits should I be looking at?

 

[pstamler]

[quote author="tablebeast"]PRR, thank you for replying kind sir! This is what I was getting at. It seems like I am way off trying to use the 1510, so steer me in the right direction and I will rename this thread.

I pulled one of the transformers from the unit and checked it out in a separate circuit and really liked the sound. In the Grommes it doesn't sound nearly as good, so I assume it is the Grommes Electronics that are sucking. I'm not familiar with the opamp in channels 1-4. It is an RC4156 and here is it's data sheet: http://www.datasheetcatalog.org/datasheet/fairchild/RC4156.pdf

It doesn't look too terrible on paper, but I would like to improve it still.[/quote]

It is pretty terrible, though. The slew rate is low, and the input voiltage noise density is just under 10nV/rtHz, or about the same as an OPA604. The Input current noise density is lowish (about .1pA/rtHz); the two taken together suggest an optimum driving impedance of 93k, which is quite high.

This seems to be a version of the RC4136 chip, which was never any great shakes. The 4156 data sheet boasts that it's a big improvement over a 741, which is sort of like saying you're a nicer person than Hermann Goering -- not much of a strain.

I think the input transformer is 600:10k so what kind of circuits should I be looking at?

If the mic impedance is the more usual 150 ohms, then the output impedance will be about 2.5k, which would work okay with an LT1028a opamp.

Peace,
Paul

 

[MartyMart]

Tablebeast, I just finished a pair of pre's based quite closely to the 1510
data sheet and using 1646 chips for balanced outs.
It sounds VERY good, line inputs follow the data sheet with 5k1 r's direct to
the chip inputs ( IIRC )
I'm quite impressed with them, though they are still "raw" circuit boards and
not boxed up as yet.

I would be happy to use them "as is" so unless you HAVE to use the iron, it
is a high quality and relatively simple way to go. ( you have the chips right?)
At the moment I have standard gain pots, but will replace them with 12 step
switches before putting them in an enclosure.
It will be one of tat's 4x mic pre boxes and the other two will be green pre's.

MM.

 

[okgb]

I have a gromes tube job which i am in the process of auditioning
different one tube pre designs , the mods of the one in it which feeds
and eq stage with make up and a output driver are not enough by
themselves , gainwise .
the nyd one bottle version is pretty good , only so much gain you can get out of a one tube but the idea was to wire direct outs and make use of
the gromes in a greater way , anyone have good one tube ideas ?