TBG E601 modules

[mata_haze]

HEllo everybody,
I just scored a couple of the above and I am racking them up right now BUT...
they are unbalanced output and I was wondering if any of you has ever hooked a transformer to balance the output and added an output trim..



http://img196.imageshack.us/i/wwwkubarthde.pdf/



any suggestion or hint to get me started will be much appreciated.



Best Regards,
Mattia Sartori.

 

[moamps]

Hi,
I've built one of those.

The transformers were salvaged from a Siemens 1-6 line distribution card, but I forget the type. Anyway, I believe any line output transformer (Pikatron, etc.) from Neumann, Lawo, Siemens distribution cards would work just as well.

A friend of mine now uses the preamp for drum section mikes mostly and is very pleased with the results. There wasn't any need for setting up output gain control because input gain is widely controllable, and gain of the output amplifier can be adjusted within the 10dB range. The modules have been slightly modified (Test gen switch is used to engage phantom power).

Additional pics can be found on my website.

Regards,
Milan 

 

[mata_haze]

that is great!!!
so the output pair of transistor can effectively drive a trafo.
very good.
the reason why I wanted an output trim was so I could LOAD (saturate) the input stage and then attenuate the output and hit my converters at a good level without distortion.
I am not familiar with these units so I would not know how they sound lie when driven hard, but I thought the option would be good.
again, where is this trimmer located?

Best,
Mattia.

 

[moamps]

again, where is this trimmer located?

Hi,

If you are referring to the output preamp gain pot, it's the red knob on the module. The pot is labelled R1 (4k7) in the schematic.
An additional gain control pot (100klog or more) can be inserted in the input of the output stage (between point A and capacitor C1 of the output amp).

Regards,
Milan

 

[abbey road d enfer]

I wouldn't count too much on the output stage to drive nicely a transformer without checking a couple of things. The low value of the output cap (C5) may create a large LF bump if the transformer has low primary inductance - which is the case of many commonly available (Sowter, OEP, Jensen). Increasing the value of C5 may solve the LF problem but the push-pull stage itself may have difficulties driving a low inductance xfmr. Be prepared to spend some dough on the trannies.
And there is something I really don't like in this preamp: the low-cut filter uses the input xfmr's inductance as an element of a 2nd order hipass, which seems fine at first sight. The idea is that offensive LF are filtered out even before entering the xfmr; but this fine scheme is put down by the fact that the xfmr sees a very high source impedance at its corner frequency, yielding high THD figures. So you use the filter to clean up the LF and you end up muddying the low-mids.
This is a part of a Teutonic broadcast mixer. These guys are concerned by commissioning figures only, and they have plenty of time to spend making sure their products measure well (who needs to spend this amount of complexity and expensive switches and adjustments to make sure the gain is accurate within 0.1dB?), but this is very often detrimental to the real world operational performance.

 

[moamps]

I wouldn't count too much on the output stage to drive nicely a transformer without checking a couple of things. The low value of the output cap (C5) may create a large LF bump if the transformer has low primary inductance - which is the case of many commonly available (Sowter, OEP, Jensen). Increasing the value of C5 may solve the LF problem but the push-pull stage itself may have difficulties driving a low inductance xfmr. Be prepared to spend some dough on the trannies.

Hi,

I didn't notice this bump in my preamp when I was doing final measurements.
If there was any bump, however, it must have been below 20Hz where the input transformer isn't linear anyway.
But, granted, it's a cheapo module we're talking about here (around 150 euros on Ebay) so it's quality should be viewed in this context. In my experience, it's good value for money.

Regards,
Milan

 

[salomonander]

hey
id like to bump this thread. im racking the same modules at the moment. i tried to put a transformer after the preamp but no luck here. i had a huge low frequency boost.
i can run them unbalanced fine but id prefer a transformer output - also id like to get the level up somehow (its at -14db now). im simply afraid of clipping the preamp trying to get a proper output level.

in my desk the modules are followed (after eq and insert points) by an output amplifier. now im wondering if i could use them to get more level as well as to drive an output transformer (schematic attached)? if yes, how could i bypass the output fader (basically have it operate at max)?

cheers


ps: one more thing. the preamp schematic wants 19v input. but the voltage regulator in my desk gives 21v. is this something to be concerned about (running it with 2v more than suggested)?

 

[salomonander]

anyone? 

 

[PRR]

> schematic wants 19v input.

Says 21V.

> i had a huge low frequency boost.

The schematic you posted shows output cap C8 as 1,000uFd. (Abbey cited a C5, which suggests he was commenting on a different plan.)

1,000uFd should be flat far below 20Hz.

It would help a _lot_ to say what transformers you used and how-much bump you measured with what loading.

If C8 is very-very-very sick, dried-up to 10uFd, you will have bass trouble. Tacking a 1,000u across it will confirm (or refute).

> get the level up somehow (its at -14db now). im simply afraid of clipping the preamp trying to get a proper output level.

Afraid?? Clip that thing! Can't hurt the circuit.

The schematic you posted can drive 200 ohms to about 5V RMS. Has gain of 15, so needs 0.3V input.

(R1 lets you trim gain down to unity.... it should be set near-max, a bit back on one to match L/R pairs.)

Should be little problem finding 0.3V to drive it.

5V should do fine work all around the studio. Leaves 16dB headroom above 0.775V, so you don't really want to meter for +4dBu (only 12dB headroom!). Metering for -10dBV (Tascam) leaves a very ample 24dB headroom.

A custom 200:600 transformer only gives voltage-gain of 1.73, so 8.6V max out, or another 5dB. That barely gets you to +4dBm territory. A "150:600" with light loading (>5K) also gets you there, but verify the high-frequency wiggles with such loading (5K and 1Meg).

>> schematic wants 19v input.
> Says 21V.

The limit appears to be C4 which runs 0.6V below point A which is nominally half-voltage. So on first approximation the maximum supply voltage is 2*(10V+0.6V) or 21.2V. The half-voltage may differ, R2 seems to be selected, but in any case C4 is run way too close to the edge. Unless it is super-vintage, I'd replace C4 with new 16V-25V unit. C4 is critical to sound (gain and distortion) so if in doubt rip it out. C2 could also go next-higher voltage rating. C7 is good to over 25V total supply, but is essential for getting large output in low load impedances.

 

[salomonander]

hey PRR

thanks a lot!
this sounds great. when talking about the 19V and my concern of clipping i was refering to the schematic in post one. the one of the micpre. the schematic i posted is the output amp in the channel strip that follows micpre and eq in my desk. i did not use them in my first racking attempt. i went from the micpre (in post one) straight into the transformer when i got the big bass build up.
but if this output gives me 15db plus is able to drive a transformer i will just put it between the micpre and transformer. this way i could gain my missing 14db (coming from micpre) and drive a transformer! a 1:1 should do just fine then i guess.

jakob

 

[salomonander]

hey
i did some measurements today. i dont have a proper software so i used the calibration function in room wizard eq.
weired thing is that the bass response changes depending on how i set the trim on the output amp. the lesser gain i give the more bass bump i get. i attached some graphs with full gain on the output amp, lowest gain (huge bass buildup) and the output amp and the setting i found most flat. i also have a graph for the flattest setting with transformer attached. this bumps the bass a little it seems.

what could be the cause for this hughe bump when minimum gain is applied?
also, any idea why the preamp has this slight HF bump? the 15k trim in the micpre module is set to minimum. and i thought that broadcast modules roll off after 14k... certainly does the opposite here (my AD/DA is ruler flat).

thanks a lot

 

[salomonander]

max gain on output amp

 

[salomonander]

minimum gain on output amp

 

[salomonander]

best setting with transformer attached