op amps into transformer question

[Guest]

i have built mixer this week with no knobs or switches, only 12 balanced inputs using ad2141 bal line receivers, summed through 10k resistors into a burr brown 604 with a 10k feedback resistor, then back out through a transformer. some channels are hardwired in L or R(for any stereo pairs--guitars, overheads, room etc..) and some go straight down the middle(for kicks, snares, lead voc). i am testing whether i like summing the digi 888 outputs externally, then dumping that mix back in(reconverting obviously) for tweaking and burning cd's.

my first day of experimenting i tried several new things. before i would always put a cap after the op amp, but i tried today without the cap, and i think i like it better, not a drastic change though. i am using a "supposed" high quality transparent transformer by audiosar. the guy that makes them said i wouldnt really hear it in the circuit. (so why did i pay 40 bucks a piece?)

i needed a way to monitor the stereo outs and simultaneously send the mix back into 2 inputs into protools, so i was looking for a dual output transformer. i called this guy near me in los angeles at Cinemag. he has transformers in various circuits on the web and people have high opinions of them. i explained to him that i wanted to come get a dual output version, but that i wanted to "hear" them. he made a recomendation, so they will be ready today and i will get them and test them. my question is can you drive a transformer(150 ohm if i am remembering correctly--but i am hooking up 2 windings in parallel so it might double) straight off the 5532 or opa 604? furthermore, what are the caps really doing in circuits showing them after the op amp? i assume blocking dc. i am looking for way to make my system sound like it does when i bring up a mix on a j series ssl. considering the j is moving faders, and when the master fader is full up, there are no vca's in the path. that leaves the inputs circuitry, the summing circuit. i am not tryng to fit the ssl stereo compressor into the equation yet.

my nest text might be to lose the ad2141 chips, and replace them with a tranmsformer input. i feel like half of what you hear with the classic old stuff is transformer color. i am not sure yet what percentage comes from the op amp.

so far the effort is paying off. any opinions or advice would be great. thanks

 

[Samuel Groner]

what are the caps really doing in circuits showing them after the op amp? i assume blocking dc.

DC blocking is a good guess. DC causes higher distortion with some trannies (depending on core material and other things). As you want to hear the tranny, you can skip the cap.

i am looking for way to make my system sound like it does when i bring up a mix on a j series ssl.

I'm not sure whether trannies are a good way to get to a SSL sound. They do not use "iron", do they? Rather clean stuff, no?

Samuel

 

[Kev]

[quote author="originator"]i have built mixer this week with no knobs or switches, only 12 balanced inputs using ad2141 bal line receivers, summed through 10k resistors into a ....[/quote]

I think you should have used 2143 and I don't think I used 10k on to the buss.

Mine was all PCB and did have space for volume and pan with either jumpers or 25-turn-trim-pots or external front panel switch or pots.
Power trafo is onboard.

It has unbal and bal out on LM833 and 2142 chips.

Two versions 12 and 16 ... I think ? perhaps I did an 8 too.
I'm sure there was an unbal input version for keys ... LM833.


What is your primary (get it ?) reason for wanting the trafos on the input ?

 

[mathtracks]

my question is can you drive a transformer(150 ohm if i am remembering correctly--but i am hooking up 2 windings in parallel so it might double) straight off the 5532 or opa 604?

If you can hold the dc offset voltage to 1mv or below you should be able to drive the tranny straight from the i.c. with good results.You may need to build a DC servo circuit if offset is not held to or below 1mv.

Here is what Bill Whitlock (President of Jensen transformers)wrote me when I asked if a TLO71 could drive a tranny:


The output transformer can be driven from the SSM2142 outputs without
problems. The SSM2142, although it has other limitations, is a far
better
line driver than a TL071. The TL071 has very high output resistance
(making
it unstable with capacitive loads like long cables) and very limited
output
current (leading to distortion of high-level, high frequency signals
like
cymbal crashes). If the TL071 were replaced with a more suitable IC,
such as
the Analog Devices OP275 for example, then it could directly drive the
output transformer.

Our JT-11-FL or JT-11-FLPCH would be a better choice for the
transformer.
The coupling capacitor can be eliminated only if the DC offset voltage
on
the transformer primary can be held to less than about 1 mV. If the
servo
holds the voltage under this limit, I'd recommend eliminating the
coupling
capacitor.
Sincerely,
Bill Whitlock, president
Jensen Transformers, Inc.


originator can you post a schemo of your mixer so we can ch-check it out.?
or can you point me to shemo of an active summing mixer?anyone?

 

[Kev]

Hi Bill,

nice to see you.

 

[Guest]

i havent drawn the schemo, but it is pretty much the standard as far as what i can tell with op amps. you simply take in your balanced input through whatever you are going to use to "unbalance" it and make it one signal(i.e. a transformer or balanced line reciever, etc). then you decide whether you will use switches for mutes, L-C-R or not. in my case, today i finished my 16 in 4 out box using no switches. there are some inputs that are stereo pairs, and some that go through on both left and right(kick/snare/lead voc etc). i use 10k summing resistors for each bus. i have seen 5k, 10k, 15k, or 20k used in various schemos before, i dont know theory much, but if you want unity gain then just use whatever value for the bus resistor on the feedback resistor and you are fine.

i went to day to seen tom richenbach at cinemag. i got several line input tranformers and a few 4 winding transformers from him. i want several windings on the output since i want to take one stereo pair and go back into protools, and another to send to my monitors. i am also using analog devices ssm2141 line receivers for some of the inputs as well. i intend to AB tomorrow to see what i can deduce with the tranny/ic inputs. so far the thing is sound a lot better to my ears than mixing in protools alone. if you still need it i'll try to draw the schemo.

someone asked why transformers: mainly i like experimenting. i think in some cases the sound of a gadget is derived from the transformers, depending on what you use. i just want a sound that has a lot of detail, with some color added that is pleasing to my ears. i think i found it with the cinemag stuff. tom says the inputs i bought are fairly transparent, yet the outputs will add some coloration. i have used the ad2142 before for outs and thought they were rather bland.

alli can say is, i pulled upo a mix i did a few weeks ago strickly within the prools, and i spent not even 20 minutes on a new one with my summing amp and i am pretty embarrased by what i did a few weeks ago without it. rather bizzare since the difference isn't radical on a per channel basis.