[BUILD] fripholm's TG1 Zener Limiter boards - support thread

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warpie

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If you're using this transformer in 2:1 mode (-6 dB) you don't have to change the values from the schematic. 2:1 will be in either series-to-series or parallel-to-parallel configurations. I don't have experience with this transformer in the TG but I'd think it could be a nice fit.

Hi Fripholm, I'm not sure what values from the schematic you're referring to. My question is about the resistor that has to be connected on the secondaries of the input TX in order to set the input impedance of the unit. I'm probably missing something but I can't find it anywhere in the schematic...

I'm thinking of wiring the 9046 as series:series (10k:2k4). I don't think there will be any benefit in using the parallel:parallel (2k4:600) configuration.
 

fripholm

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I'm not sure what are you referring to. I thought you meant the BYPASS resistor divider consisting of R86/R87...

When wired correctly, there's no need for an additional resistor on the secondary of the I/P transformer. Depending on the BYPASS switch's position, the transformer always connects to roughly 3 to 4 kOhms which is the input impedance of either the compressor stage or the output stage (when in BYPASS). With a 2:1 winding ratio the input impedance to the outside 'world' is four times that - therefore about 12k.
 

warpie

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I'm not sure what are you referring to. I thought you meant the BYPASS resistor divider consisting of R86/R87...

When wired correctly, there's no need for an additional resistor on the secondary of the I/P transformer. Depending on the BYPASS switch's position, the transformer always connects to roughly 3 to 4 kOhms which is the input impedance of either the compressor stage or the output stage (when in BYPASS). With a 2:1 winding ratio the input impedance to the outside 'world' is four times that - therefore about 12k.
OK I missed this bit. Thanks a lot!
 

Juanaca

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Hello colleagues, I am joining this interesting project, everything is more or less clear, I just need to find out about the audio transformers,Some speak of 2: 1 in input and 1: 1 in output without talking about impedance, others say that 10k: 10k in input, but this is 1: 1, right?The only thing that I have clear is that the output must be 600: 600.any clarification is appreciated.thank you very much !!
 

JMan

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You can trawl this thread for more in-depth discussions of this, and I'm sure fripholm will chime in with something helpful, but I think the short answer is this:

Input = 2:1 -- can also be higher (e.g. 4:1) but this will require some changes to a few resistors on main board
Output = 1:1

Impedance of input xfmr is not crucial here (but may theoretically have a small impact on frequency response) except as it relates to ratio.

Anybody, please correct me if I'm wrong!
 

fripholm

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Transformer impedance given in their specs is mostly *rated* impedance and is pretty meaningless on its own. It does not mean that a specific transformer can only be used at this particular impedance. Look for a 1:1 transformer that is specifically made for line outputs. Although I've not tried it, the VTB2281 from Carnhill looks good. You could wire both the primaries and secondaries in series or both in parallel and see which one gives you the better sound, if there's a difference at all...

As we've learned from Winston OBoogie the other day, the original was supposed to have a 1:2 output, which the VTB2281 can do as well. In this case you have to make sure the load impedance doesn't get too low. Line inputs on most if not all modern gear will be perfectly fine, though.
 

warpie

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What the output impedance of this compressor would be in a 1:2 configuration?
 

Juanaca

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Transformer impedance given in their specs is mostly *rated* impedance and is pretty meaningless on its own. It does not mean that a specific transformer can only be used at this particular impedance. Look for a 1:1 transformer that is specifically made for line outputs. Although I've not tried it, the VTB2281 from Carnhill looks good. You could wire both the primaries and secondaries in series or both in parallel and see which one gives you the better sound, if there's a difference at all...

As we've learned from Winston OBoogie the other day, the original was supposed to have a 1:2 output, which the VTB2281 can do as well. In this case you have to make sure the load impedance doesn't get too low. Line inputs on most if not all modern gear will be perfectly fine, though.
friphold thanks for the answer, I am clear that the output transformer must be 1: 1 600: 600, but the input transformer is what I doubt, it has to be 2: 1 10k: 2k ??
sorry for my persistence but the topic of transformers short circuits my brain.
Thank you so much
 

fripholm

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What the output impedance of this compressor would be in a 1:2 configuration?

Don't know exactly, but should be less than 100 ohms.

it has to be 2: 1 10k: 2k ??

As I said before, these numbers do not really matter. A 2:1 transformer has an impedance ratio of 4:1, so with a 10k:2k you're in the ballpark.

For the input, you'll need an audio transformer that is suitable for line inputs that has a turns ratio of 2:1. In an earlier email you asked about Carnhill's VTB9046(M) which is about perfect for this job. I'm pretty sure this one was mentioned a lot within this very thread (but not exclusively).
 

warpie

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Don't know exactly, but should be less than 100 ohms.



As I said before, these numbers do not really matter. A 2:1 transformer has an impedance ratio of 4:1, so with a 10k:2k you're in the ballpark.

For the input, you'll need an audio transformer that is suitable for line inputs that has a turns ratio of 2:1. In an earlier email you asked about Carnhill's VTB9046(M) which is about perfect for this job. I'm pretty sure this one was mentioned a lot within this very thread (but not exclusively).
Thank you fripholm. It was just curiosity. One last question. Do you suggest using a load resistor across the secondaries of the output transformer and if so, what would be an appropriate value?
 

fripholm

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You might not need one at all but that depends entirely on your transformer. Try with (different values) and without and see what happens. Experiment! :cool:
 

fragletrollet

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Hey,

I have the 2.1 boards. Why is r15 followed by an "*" after it on the silkscreen? On the BOM it looks normal/mandatory.
 

fripholm

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This is a leftover from the original TG schematic where this was an ADJUST ON TEST resistor. You'll find more of those on your board (R7, R8, R39 etc.). Apart from the ones mentioned in the guide for calibration you can ignore those.
 

freshfromtheden

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Just assembling the parts for this and, I don't think this has been asked here yet - do I want MBB or BBM rotaries for the attack, sidechain and mode switches?
 

freshfromtheden

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Just assembling the parts for this and, I don't think this has been asked here yet - do I want MBB or BBM rotaries for the attack, sidechain and mode switches?
Thinking about it, these are just CVs I guess so it probably doesn't matter? We'd only worry about using MBB on the audio path to deter popping when switching?
 

Juanaca

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Hello, reviewing the guide I have a problem of understanding with the part of the R7 and R8 calibration.
I understand that you have to put a trimpot to find the right ohm measurement for compression, but what about R8?
I can't understand the example value of R8 and 82k?
Thank you so much
 

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JMan

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Hello, reviewing the guide I have a problem of understanding with the part of the R7 and R8 calibration.
I understand that you have to put a trimpot to find the right ohm measurement for compression, but what about R8?
I can't understand the example value of R8 and 82k?
Thank you so much
R7 and R8 are in parallel, meaning that you have to calculate their values accordingly. In the build guide example, the measured value of the trimmer after fine tuning is ~3.7k. Thus, the example of a 3.9k and a 82k resistor in parallel is used to achieve the desired value.
 
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