THAT4301 compressor

ungifted · Apr 8, 2013

I've read some topics here, concerning THAT4301 compressor design but couldn't find any finished project.
Though it would be interesting to build something like you-know-what-I-mean compressor.

The scheme is in attach.
Will this scheme work?

Well, there is no info but the THAT notes and datasheet in the scheme.

Basic schematics is from Demo Board Datasheet Demo Board Datasheet and DN115. Peak/RMS is from DN101.

I have some questions:

OA2 which is control voltage buffer. Are there some tricks around Peak/RMS switch (which added R20 - 45k3), R19 (in various datasheets I can see 10k, 20k and 30k) and feeding control voltage into EC- of the VCA?

Is everything OK with Hard/Soft section?

And to sum up - is there any interest in a noob friendly compressor based on THAT4301?

I'll be thankful for any support.

hazel · Apr 11, 2013

Hi ungifted!!!

You've read my mind. I'm planning my 4301 compressor based on DEMO BOARD and i was just figuring out which bells and whistles to implement:
- stereo compressor, no dual mono/link switch
- auto/manual mode switch
- selectable HPF on sidechain (maybe 80, 100 and 200 Hz)
- peak/rms mode switch
- hard/soft knee mode switch
- rotary switches (12 steps or less) for ratio, attack and release controls

I was planning to mix some app notes from THAT page

I'm with you man!!!!

gemini86 · Apr 11, 2013

I know there isn't an open source project here for it, but i thought I'd mention the SCA B16 compressor kit. It's very well priced for what you get. I feel like nobody knows about it.

njm · Apr 11, 2013

I'd check the app notes. There's a lot of info on the THAT website on how to use this VCA/RMS chip

Not sure about that rms/peak switch you've got. The 4301 has a RMS detector, it does RMS.

Varying the cap on the Ct pin wont vary it between peak and RMS, it will just leave ripple on the control voltage

hazel · Apr 12, 2013

gemini86 said:
I know there isn't an open source project here for it, but i thought I'd mention the SCA B16 compressor kit. It's very well priced for what you get. I feel like nobody knows about it.

It seems a good compressor but i don't want to invest in custom chassis an psu. 160 dollars each compressor, 270 dollars chassis and 85 dollars psu, that's 600 bucks for a kit, doesn't have gr meter and what i do with empty slots?

hazel · Apr 12, 2013

I really prefer DIYing or starting with DEMO BOARD

njm said:
I'd check the app notes. There's a lot of info on the THAT website on how to use this VCA/RMS chip

Not sure about that rms/peak switch you've got. The 4301 has a RMS detector, it does RMS.

Varying the cap on the Ct pin wont vary it between peak and RMS, it will just leave ripple on the control voltage

You're right, i think i'll pass on peak/RMS mode, it'll be interesting HARD/SOFT switch and HPF on sidechain,
How feasible would be? Just inserting 2 or 3 passive HPF before RMS detector selectable with rotary switch?

ungifted · Apr 12, 2013

Thank you all for your answers!

Yes I've seen the SCA B16 schematics, It's all the same (plus minus) from datasheets.

The main question for me is the Peak/RMS switch. Does we need it? Peak detection is described in DN101
http://thatcorp.com/datashts/dn101.pdf and pico has it.

njm

THAT4301 datasheet says that the cap on the Ct is the timing capacitor:
"The detector, which normally responds in true rms fashion,
is re-configured for peak operation by making C1, the timing capacitor, quite
small, thereby disabling the logarithmic filtering." (DN101)

leitmo

I'd rather keep things simple, make project which is suitable for noobs who need simple diy compressor project.
Sure we can try to do every item from your list, let's start with basic design, and add more features as we finish the previous.

What do you think?

ungifted · Apr 12, 2013

Here is the lib for Eagle with some updates: it has VCA, OA and RMS detector symbols - so we can draw more intuitive version of the scheme.
I think It's usable for schematics drawing only, I'm not sure about using it for PCB design in Eagle CAD.

hazel · Apr 12, 2013

ungifted said:
The main question for me is the Peak/RMS switch. Does we need it? Peak detection is described in DN101
http://thatcorp.com/datashts/dn101.pdf and pico has it.

leitmo

I'd rather keep things simple, make project which is suitable for noobs who need simple diy compressor project.
Sure we can try to do every item from your list, let's start with basic design, and add more features as we finish the previous.

What do you think?

Peak/RMS is a great feature, studying app note 101 seems easy to add but...would be real PEAK detection?
Yeah, simple things are always a good idea.

My first intention was to get a couple demo boards and measure ATTACK and RELEASE "as is", in AUTO mode, and then add manual control, it's not that hard to add....looking at app notes

So, your main features are: peak/RMS and hard/soft switch?

ungifted · Apr 12, 2013

Yes main features are from DN115 http://thatcorp.com/datashts/dn115.pdf plus optional Peak/RMS.

Variable Attack/Release as option (it can be even on separate PCB, like GR meter or balanced IN and OUT with relay bypass).

So we have a heart of project - THAT4301 + 3 knobs (Threshold, Ratio and gain) plus two switches - Peak/RMS and Hard/Soft.
I have some PCB sketches made in eagle, with pots on PCB and switches offboard. It's 80mm in width so it's possible to mount it in 1u horizontally and in 2u - vertically.

What do you mean by saying "real Peak detection?"
DN101 describes the approach...

hazel · Apr 12, 2013

ungifted said:
What do you mean by saying "real Peak detection?"
DN101 describes the approach...

i was just asking, i'm not an engineer and my knowledge is limited. You're one or two steps beyond me

I like the separate PCB's approach.

Any idea for GR meter? i really like this:
http://www.jlmaudio.com/shop/index.php?_a=viewProd&productId=100
IN / OUT / GR meter
I don't think i'll use IN and OUT meter so 40dB meter is too much for GR, i use to apply just a few dB's, maybe there's a chance to adjust 0'5dB per LED

ungifted · Apr 12, 2013

GR meter is described in http://thatcorp.com/datashts/dn112.pdf
And I've seen a quite simple one led feature in datasheets.

JLM type is good but to my taste is too complex for our project. We can use nearly any similar led meter, need to adjust for control-voltage in our schema only...

Can anyone post a rough drawing of peak/rms switch (threshold OA) and a gain buffer?

In the posted schema I suppose some mistakes in gain buffer: we have voltage drop on R20 (45k3) and R19 (10k-30k) and when we feed control signal to negative(!) input of the OA2 must we change R24?

When we use positive input of OA2 and then feed control-voltage to VCA's EC+ as I understand there's no problem (it's right from datasheet, and the signal level is right) but is there any way to use EC-? Help needed.

PS: Sure we can use EC+ as described in DN101, but feeding signal to EC- allows to connect pins of THAT4301 on one layer PCB with the shortest tracks...

usekgb · Apr 12, 2013

Search for the DBX LED gain reduction meters in the DBX schematics page. They're fairly simple.

Bill Wilson · Apr 13, 2013

KA Electronics used to have pcb's available for a gain reduction, level meter based around an LM 39xx chip. The meter was developed for the pico compressor & there used to be pcb's for sale at KA Electronics. I will check the pro audio forum to see if they are still available. They would be perfect for another 4301 compressor build.

ungifted · Apr 13, 2013

I have all docs from KA, and PCB. I think if we can find a lot of similar gr meter projects.

GR meter is not the main problem.

The question asked in my previous post (gain buffer) is still actual. Any ideas?

hazel · Apr 18, 2013

Hope more experienced people could help enlightning us about gain buffer

I cannot find attack and release times on 4301 demo board and basic compressor app notes, any info/clue?

ungifted · Apr 18, 2013

I've changed the schematics, oa2 in gain stage is non-inverting repeater now. Working on a PCB.

Attack and release option is described in DN115.

hazel · Apr 18, 2013

i know about app note 115 "fully adjustable compressor/limiter" and attack and release option

I was talking about attack and release times on DEMO BOARD or "Basic compressor" Design note 00A, the one without Attack and Release pots, just Threshold, Ratio and Output.

ungifted · Apr 20, 2013

Last version of schema and PCB design.
Сomments are welcome!
PS: Some minor changes are followed.

gemini86 · Apr 20, 2013

Just a suggestion, but if you're going through all the trouble to make a PCB, add a balanced output. You won't regret it.

THAT4301 compressor

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