amp for NYD passive Summing box

Gentlemen, this is a "try" to drow an amp for gainup the 8 ch balanced NYD passive box, using the neuman OA10.( I 'got some) I've used the design of a mic pre found in the tecnical paper of the Neumann AO10 as tipical applications, as fisrts stage and second one a buffer out with a small EQ.

first question is if this value are right or not, and if all this does make sense.

I have thought to use an OEP 1:6.4+6.4 as an input; if so, at the sencondary of the trasformer patched in series, I 'll find about +20db right ?
so I will need 24 db more by the input op amp to compensate the 40 db lost of the passive summing, right?

the fisrt OpAmp got to gain about 50 time. witch resistor I've to change R5 ?

do you think is possible to use one half of the secondary of the output trasformer to go to a recording machine, and the other half to drive an amplified control room monitor pair?
in this case I've to add a pot after the secondary or a buffer is needed ?

thank you
M.

Just a note: I've posted a few ideas for balanced passive mixing networks here. But here's the version I'm now recommending in the usual case where the makeup gain is provided by a mic preamp:

PDF

yes your design have inspired me, just want to know if mine can do the job :roll:
M.

R9 can be much smaller, say 39 ohm (or probably skipped entirely). Doing this will reduce output transformer related distortion dramatically.

R5 is the easiest way to modify the make-up gain. You probably want this to be variable, i.e. with a rotary switch.

Samuel

I presume that the Neumann OA10 is self biasing.

Not so sure about using that transformer at the input.... the OEP type shown is reasonable, but a little droopy at the bottom end; and not exactly flat above 8K.
Samuel is absolutely right on the output transformer, but here again, if it is driven from zero impedance it will behave extremely well except that the core will saturate at about 0dBu at 25Hz (from memory) so keep the levels down! :wink:
Driving the transformer via a series resistor will 'soften' the satuation, but will, as Samuel says, show up some inherrent distortions in the transformer. It's worth trying it with 150 ohms as a compromise.

thank you gentlemen.

@ Samuel : YEs I will use a rotary switch for the gain, the Hig the value of R5 the smoler the gain Make up, right ?

The AO10 's got about of about 250 ohm output, and can drive 200 ohm so I will schort out R9

@ TedF : yes AO10 is a self biased op amp, I can use a different type of input trasformer, probably a Cinemag 1:2 or 1:4
the output tranny is the CMOQ-2L.


the question is about the secondary of the out trasformer, May I use the second Half to drive an amplified monitor stere o pair, what's the trade of ? I plan to use it as indipendent control
Will I need some more current ?
M.

The higher the value of R5 the smaller the gain make up, right?

Click to expand...

Yes, the formula is:
gain [dB] = 20 * log ((R4 + R5) / R5)

I don't think the second amp ever sees 200 ohm to drive--input impedances for line inputs are around 10k nowadays.

Having a small resistor before the transformer isolates capacitive loads which otherwise could cause instability. If you design a PCB, I would include R9, short it first and see if you encounter any stability problems while driving a few dozent meters of cable. If you do, replace the link with a small resistor.

Unfortunately I never studied EQ designs yet so I cannot tell you at which noise gain the second opamp runs (the higher the noise gain, the better the stability).

Personally I'd make C1 a tad larger, say 2200 uF.

Samuel

ok, from 39 to 150 ohm strarting point for R9, and try different value for C1, thank you.

R6 can be smaller the 600r say 300r I guest, R10, 11, 13 are 600 but i need a 6db max increment/decrement for each freq.

what about the output issue ?

The output is no problem; as long as you remember that the load on the transformer is a simple sum of the 2 destinations.
It will be fine if both the outputs are driving into 'bridging' loads (greater than say 10K ohms).

What about the output issue?

Click to expand...

I don't see any specific problems by splitting the secondaries--but what transformer are you using?

A gain control after the transformer is IMO not a good idea--you'd drive a cable with rather high impedance at medium gain settings.

I just realised that you want to build a 8 channel unit--that makes 18 dB loss, not 40 dB...
[Edit]: I see that NYD suggests 40 dB loss. I'd do it differently if you've got a built-in make up gain section.

Samuel

Dave,
Can you please give an explanation of the math behind your solution (or point me to a thread if you've already explained this)?

Many thanks,
Frank

> 1:6.4+6.4 as an input; if so, at the sencondary of the trasformer patched in series, I 'll find about +20db right?

Voltage gain from primary to series-secondary will be 1:13 or almost 23dB. Maybe not quite 22dB in real life.

But the first opamp seems to be runing at gain of 22,043/43 or 1:512 or 54dB. Second opamp works at unity gain. 22dB+54dB+0dB= 76dB. 40dB mix-loss, you still have gain of 35dB from mike-amp output. This seems like far too much, for an amp with no internal gain control.

For unity overall gain, 40dB-23dB= 17dB gain needed in the first opamp. I think the "43 ohm" resistor should be more like 3K6, and its capacitor reduced accordingly.

For pop music mixing, unity gain from many similar-level inputs makes an output larger than any one input. One rule of thumb might be 3dB for each doubling of inputs. 2-in, 3dB, 4-in, 6dB, 8-in, 9dB. Then you only want 17dB-9dB= 8dB gain in the first opamp. Or if you figure only half your inputs are "stars" and full-level, 17dB-6dB= 11dB gain needed. This suggests the "43 ohm" should be in the area of 10K.

ok, gentlemann, assuming I'll use this input trasformer I've corrected some value, and I've added a variable gain control. R16 shold be 18K I Guest. C1; C2; C8;C7 are still approximated .
I've redrow the out put trasformer ( cinemag CMOQ-2L ).
Now the 2 secondary are in series 1:2 ad the control room buss is taken from the second Half of the primary.
It seams to me that the cheper way to do this is probaly buffer this buss by another op amp or a bridged-T attenuator.

shold it be ?



Thank you

Your image is not showing for me.

Why did you split the primaries and not the secondaries of the output transformer?

I'm not happy with R15; this is a cruel way to set levels. I don't see a simple way to add a gain level that works very well, but two series resistors (say 2k2 each) after the transformer output windings and then R15 (5k) might do it.

I would think about rising the impedance level of the EQ a bit, say to 2k2 instead of 600 ohm. Makes the second amp work less hard, and noise isn't an issue here. You need different inductors though.

Samuel

Your image is not showing for me.

Click to expand...

same here!

It's a CMYK JPEG--too much for IE and Mozilla as it seems.

Temporary fix: [removed]

Samuel

[quote author="Samuel Groner"]Why did you split the primaries and not the secondaries of the output transformer?

I'm not happy with R15; this is a cruel way to set levels. I don't see a simple way to add a gain level that works very well, but two series resistors (say 2k2 each) after the transformer output windings and then R15 (5k) might do it.

I would think about rising the impedance level of the EQ a bit, say to 2k2 instead of 600 ohm. Makes the second amp work less hard, and noise isn't an issue here. You need different inductors though.

Samuel[/quote]

thank you Samuel.
Mine are just tries,i've split the primaries for make the step up secondary configutarion, I do not know if this does works or not, but pleas let me know, I think the simplest way to make a control room, is to built it as amp apart. correct me if I'm wrong.

I'm gonna try the values suggested by you, to get right tune.
thye problem to rise the impedance beetween the 2 amp ( EQ ) is that I've to build bigger indictors, but if you say this can make a bettere noise ratio, I will follow your minde.

Thank you

Just to make sure: the way you've drawed your latest schemo works well except the volume with R15. My suggestions are mostly to tweak things a bit.

I suggested a higher impedance network for the EQ as this would make an easier load for the second amp to drive--i.e. 2k instead of 600 ohm. But it works with 600 ohm, and noise is even better with the 600 ohm network. But as I said before, noise considerations for the EQ are more or less irrelevant as noise well be dominated from the first amp.

What is the max. input level of your recorder? If it's 20 dBu or less, you don't need the step up. Remember that step up transformers reflect a rather low load impedance to the driver--if your recorder has a 600 ohm input impedance, the second opamp sees 600/(2*2)=75 ohm, which is very low. Most certainly your recorder has 10k or more, so this is not a problem. Nonetheless I would prefer a non-step up solution if this gives you enough voltage drive.

Why is R15 an unhappy solution? If you turn down the volume, you short one primary--this reflects to the driving amp which sees now about 100 ohm (R4 plus both DC resistances of the primaries in series). This will lower the level to the recorder and increase distortion as well.

By inserting the two series resistor, this interaction is strongly attenuated.

BTW, I would interchange R16 and R17 (so the pot goes to C1). This helps isolating stray capacity from the inverting input which otherwise could cause instability.

Samuel

thank you Samuel, I'will back to the original secondary arrangment. and try your minde.
thank you again.
MIK :grin: