Changing summing amps

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rotation

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Jan 24, 2006
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Hi, a friend would like to change summing amps in his Gelf 24/8 70's console. He told me there are 4 TL074s as a summing amps.

I was thinking about installing API 325 amps for summing and have some questions:
1. Does 325 have enough gain for 24 channels?
2. What is 425 combining network and do i need it?
3. Would you leave the old summing amps untouched and make external summing box connected to direct outs, inserts or groups (depends if pan is needed)? In my opinion it is better to leave such a mixer (it is well done, sounds&looks good, but wiring is a big mass) as it is if it's not really necessary to change things.

For now i don't have any schematics to show you, but i checked the input module, saw some pics and got informed about designs from that time, and i can say it is very simple. Input modules have transformer balanced inputs, amp part is TL074, eq MCI458 and so on. I'm not sure what output, aux, monitor..amps are, but it seems TL074s are used everywhere and are supported with transistors in some places. All switches, pots and specially faders seem to be of good quality.

I have one more question: i searched the whole internet to find informations about this or any other Gelf mixer, designers names, but with no luck. If anyone could help us out we would be very thankful. Same goes for summing.

Miha
 
Actually, job #1 is determining why he feels the need to change the summing amplifiers. "Because they're TL074s" is not a good enough reason. Was he happy with the console before he opened it up and saw the numbers on the chips? That's the real question!
 
If the '074s in summing amps are supported by discrete transistors (in front end) the performance could already be quite good.

Perhaps run some measurements of sum amp performance and make a judgement from there.

JR
 
Thanks for the replys!
We done some listening tests and summing part seem to be ok, but not really what friend wants. So as he likes API a lot, we thought of making summing amps with it and beside for summing he could use them to warm up the signals (he does a lot electronic music, line levels mostly) and as a simple mixer when he don't need his Gelf. From my experiences preamp like 325 does some magic to digital recordings.

As the old summing is good i'm thinking of making external around 16ch summing box with 325s and use them when needed. I think i already answered myself about making it internal or external.
I would like to ask you:
1. Can 325 be used to sum 24 channels? Manual says it is ok up to 22ch and you end up with 20dB of gain. 20dB is probably more than enough for any line signal, no?
2. What could be combining network that can be a part of 325?
3. Where can i get some reading about 70's console design? I have John Robert's Designing Mixers articles (good but basic reading), Allen&Heath MOD3 manual and nothing else.
 
if you just want the 325 sound why not make an attenuator to lower gain then use the 325 to do make up gain and give that API sound. Make it patchable ten you can choose between the 2 :? might work easier then changing the summing amps of a console
 
[quote author="pucho812"]if you just want the 325 sound why not make an attenuator to lower gain then use the 325 to do make up gain and give that API sound. Make it patchable ten you can choose between the 2 :? might work easier then changing the summing amps of a console[/quote]

I was thinking of this. But is it good idea i we want to preserve S/N ratio? A frined needs separated summing box/mixer anyway, so we can connect it to dir. outs and use it as a summing.
 

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