etheory - a bunch of projects in progress

[ruffrecords]

Well, I sent two messages to THAT Corp. but look like they don't have any comment about our issue!!......Would be a good idea someone try to contact them again with the same question!!

I am very intrigued for a analysis for that analysis......

Opacheco.

I think the answer may be even simpler that I thought. I think the gain around the loop sets the threshold and nothing else. The transfer function of the gain element sets the ratio and nothing else. It is similar to an op amp with a switched feedback network. Up to the threshold the gain is one value and above it the switch gets operated and the gain changes. It does not matter if you put some extra gain after the switch in the NFB network it will not affect the threshold. I need to dig out one of my old control theory books to try to turn this into some equations.

Cheers

Ian

 

[opacheco]

I think the answer may be even simpler that I thought. I think the gain around the loop sets the threshold and nothing else. The transfer function of the gain element sets the ratio and nothing else. It is similar to an op amp with a switched feedback network. Up to the threshold the gain is one value and above it the switch gets operated and the gain changes. It does not matter if you put some extra gain after the switch in the NFB network it will not affect the threshold. I need to dig out one of my old control theory books to try to turn this into some equations.

Cheers

Ian

Ok, that will be fantastic !....I will be pending  of your result analysis.

Opacheco.

 

[opacheco]

I found these interesting LA-2A electronic functional videos, maybe someone can find some light in this AFTER/BEFORE OF THERSHOLD GAIN issue.

https://www.youtube.com/watch?v=mxAWlNs6i60&index=3&list=PLWtilvqU0-yzcmtac_Ybc9AEep_A2CCeK

https://www.youtube.com/watch?v=3QafLf70kxY&index=6&list=PLWtilvqU0-yzcmtac_Ybc9AEep_A2CCeK

Opacheco

 

[opacheco]

Dear Friends,

I am thinking all the time in the Before Gain or After Gain Threshold section; but I would like how the Neve 2254 compressor works because some people say this is a version of Siemens U273. I have revised the schematic for this Neve unit but is not to clear for me how this works.

Any comment?

Opacheco.

 

[ruffrecords]

The 2254 uses a diode bridge as a voltage controlled resistor.

I am still working on the theory but other things have got in the way - like moving house!

Cheers

Ian

 

[opacheco]

The 2254 uses a diode bridge as a voltage controlled resistor.

I am still working on the theory but other things have got in the way - like moving house!

Cheers

Ian

Ok Dear Ian, take your time......Maybe someone can help me in this Neve 2254 circuit explanation while you finish the theory about the Before/After Gain of the Threshold network.

Thanks
Opacheco

 

[etheory]

Opacheco, that is correct.

The math is pretty simple with regards to this.

ruffrecords your statement that the gain in the side-chain after threshold affects ratio only was precisely my original statement, so I'm not sure we disagree at all any more.

I understand what your spreadsheet is basically trying to say, but I couldn't get it to work in OpenOffice properly (I don't use Microsoft Office). Fundamentally, however, the speadsheet is for a feed-forward compressor.

Here is a really really simple implementation in Desmos to clarify (for feed-forward): https://www.desmos.com/calculator/oyomsskeqs

 

[ruffrecords]

Opacheco, that is correct.

The math is pretty simple with regards to this.

ruffrecords your statement that the gain in the side-chain after threshold affects ratio only was precisely my original statement, so I'm not sure we disagree at all any more.

Cool, so if you also agree that gain before the threshold only affects the threshold then we are in complete agreement!

I understand what your spreadsheet is basically trying to say, but I couldn't get it to work in OpenOffice properly (I don't use Microsoft Office). Fundamentally, however, the speadsheet is for a feed-forward compressor.

Here is a really really simple implementation in Desmos to clarify (for feed-forward): https://www.desmos.com/calculator/oyomsskeqs

I had a quiet giggle when I read you said you could not open the spreadsheet in open office because that is what I used to create it. I exported it to M$ format for posting here. By the way, it is a feedback compressor.

Cheers

Ian

 

[andreapetucco]

Did anyone tried to built the prof. Marshall Leach jfet limiter? the design is very straightforward, it uses a linearization network to reduce the second order harmonics typically produced by  a jffet.
Take a look at this:
https://leachlegacy.ece.gatech.edu/papers/limiter.pdf
I thing using a quality  opamp in A1, the result can be surprising.

 

[SirNickity]

While we're digging up old threads...  I'm actually quite interested in the U274 design that was coming along here.  I've been studying the original schematic, and the various modifications discussed earlier in the thread, and .. I think .. have a fairly complete picture of where etheory left off.  I'm going to try building one and see what color smoke it makes.

 

[opacheco]

While we're digging up old threads...  I'm actually quite interested in the U274 design that was coming along here.  I've been studying the original schematic, and the various modifications discussed earlier in the thread, and .. I think .. have a fairly complete picture of where etheory left off.  I'm going to try building one and see what color smoke it makes.

Dear SirNickity,

Please let us to know when you have some mod done or any mod idea in order to comment and add some point of view of them.

Opacheco

 

[SirNickity]

I'm not sure that mods are my specialty.  My analog design skills are meager.  I'm just hoping to bring the work already done back to life.

While I'm on the subject, one thing I'm not quite sure of:  Where would the feedback cap C5 connect to?  In the original, it's going to the middle of three pins going to the output transformer.  The other two don't look like they're being used.  The amp output connects to an outer pin on the top, "0V" to the bottom.  Is it safe to assume the middle three pins are taps along the primary coil?  If that's the case, maybe C5 would be driven by a 50% amplitude copy of the output signal?

AFAICT, this was never shown on any of the schematics etheory posted -- and on the PCB, it just connects to an unlabeled pin in a header block.

 

[etheory]

While we're digging up old threads...  I'm actually quite interested in the U274 design that was coming along here.  I've been studying the original schematic, and the various modifications discussed earlier in the thread, and .. I think .. have a fairly complete picture of where etheory left off.  I'm going to try building one and see what color smoke it makes.

Sorry everyone!

I'm still lurking, I'm just far too busy in other stuff in my life to get back to these projects just yet. Sorry!

I'm getting married in July of this year, then I'm presenting some of my graphics research at Siggraph LA in August, about my work on the render engine used for LEGO Batman (I'm one of the lead software engineers working on the Animal Logic proprietary graphics rendering software, Glimpse). Aside from that I'm starting my own software company releasing realistic audio simulations of real circuits as VSTs, using proprietary simulation technology I'm developing.

I still REALLY want to get back to this stuff, but for the moment, but sorry for the quiet.

Thanks again for the interest!

I'll help where I can....

 

[etheory]

I'm not sure that mods are my specialty.  My analog design skills are meager.  I'm just hoping to bring the work already done back to life.

While I'm on the subject, one thing I'm not quite sure of:  Where would the feedback cap C5 connect to?  In the original, it's going to the middle of three pins going to the output transformer.  The other two don't look like they're being used.  The amp output connects to an outer pin on the top, "0V" to the bottom.  Is it safe to assume the middle three pins are taps along the primary coil?  If that's the case, maybe C5 would be driven by a 50% amplitude copy of the output signal?

AFAICT, this was never shown on any of the schematics etheory posted -- and on the PCB, it just connects to an unlabeled pin in a header block.

If you look at the U274 schematic on Kubarth's site, which it sounds like you have been doing, yes, you can see that half the output signal is returned to C5. I merely simulated this attenuation with a resistor.

I found that to ensure stability and prevent motorboating, if you used 1N4148 diodes in the VCA stage, you needed at LEAST 100R for this feedback resistor from the output back to C5. If you use UC3611N schkotty diode array, then you need at LEAST 6.1K. If you are working from the original physical circuit, the return is attenuated by the output transformer and a resistor is then unnecessary.

Hope that helps!

 

[SirNickity]

It helps a LOT, man.. thank you.  I really appreciate the effort you've put in to this.  It's an interesting little box that I might never have heard of otherwise.  So please let me know if I'm stepping on your toes.  I certainly have no intention of stealing your thunder, just hoping to keep the storm brewing. 

 

[etheory]

It helps a LOT, man.. thank you.  I really appreciate the effort you've put in to this.  It's an interesting little box that I might never have heard of otherwise.  So please let me know if I'm stepping on your toes.  I certainly have no intention of stealing your thunder, just hoping to keep the storm brewing. 

No worries at all!

I definitely still intend on releasing a kit for this including PCB + parts, but that's still a few months off at best. Life is just too busy at the moment to spend as much time on DIY as I'd like to. So I'm happy for you to do whatever you like, but I will still ultimately want to offer this as a for-sale product at some point. As always, I'm still more than happy to answer any questions you like.

 

[altfidelity]

No worries at all!

I definitely still intend on releasing a kit for this including PCB + parts, but that's still a few months off at best. Life is just too busy at the moment to spend as much time on DIY as I'd like to. So I'm happy for you to do whatever you like, but I will still ultimately want to offer this as a for-sale product at some point. As always, I'm still more than happy to answer any questions you like.

Any update on this?