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Building your own stompbox / Re: BBD compander gain / output question
« Last post by Rob Strand on Today at 09:24:46 PM »
Regarding the biasing,
https://pdf1.alldatasheet.com/datasheet-pdf/view/18878/PHILIPS/NE571.html
https://pdf1.alldatasheet.com/datasheet-pdf/view/18878/PHILIPS/NE571/+07JJ9UL.hKpRudSICEY+/datasheet.pdf

Look at figure 4.  This circuit is running on 15V.   pins 5,12 have an 8.2k to tweak the bias point.
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Edit: And at the bottom of the second column on page 5:

Expandor case only:
"The output will bias to 3.0V when the internal resistors are used.
External resistors may be placed in series with R3, (which will affect
the gain), or in parallel with R4 to raise the DC bias to any desired
value."   

Compressor case is just above that:
"The values of RDC will
determine the DC bias at the output of the op amp."
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Building your own stompbox / Re: BBD compander gain / output question
« Last post by Rob Strand on Today at 09:22:15 PM »
Quote
gain= sqrt((10000*20000*0.000140)/(2*30000*0.003465)= 11,6 (+db around 20db ?)
I think you are using 30k for R3 when it should be 20k.

30k (R4) is the bias resistor and 20k (R3) is the input resistor.
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Building your own stompbox / Re: Conceptual desisn: Drone strings
« Last post by amptramp on Today at 09:21:51 PM »
Go to page 52 here:

https://www.americanradiohistory.com/Archive-Radio-Electronics/60s/1968/Radio-Electronics-1968-05.pdf

which is the article on "vibrating-wire audio filter and oscillator" and you can see how to arrange for an electrical signal from the drone wires.  It covers all the harmonics that would be present in a vibrating wire if the magnet is located properly.  You take part of the guitar output and put it through the Wheatstone bridge filter network.  The Q of a vibrating wire is quite high - over 600 at 1000 Hz.  They used two transformers but a power driver like a 386 would enable you to eliminate the input transformers and the output could be taken from a differential or instrumentation amplifier for each wire.

This is the same article I quoted for the thread on sustain generators.  The filter behaves like a pickup on a guitar - the location of the magnet determines what harmonic content you have.
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Building your own stompbox / Re: BBD compander gain / output question
« Last post by Rob Strand on Today at 09:18:21 PM »
Quote
My example is also used by Boss: 9v -> 3v bias
The other example is EHX DMM and uses -15v with bias of -9,96v

Both are not 'some people", or so I thought...
Agreed but I've see it a lot.  IIRC there's some point in the datasheet or applications notes where they talk about the bias point. [See two posts down]

There are more details consider: the gain cell and the rectifier have signal swing limits. If we just use the internal values of R1, R2, R3 and don't add external series resistors that restricts the allowed voltage swing.  However it still doesn't make sense because an asymmetrical bias might stop overload in one direction but it will make it worse in another.  I used to know all the details of those chips in my head.  Some of it has withered away.   I'm pretty sure there's no reason for the skewed biasing over symmetrical biasing.
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Building your own stompbox / Re: BBD compander gain / output question
« Last post by j_flanders on Today at 09:08:46 PM »
Some people copy the datasheet for other voltages but the biasing is stuck on 3V and is non-optimal.
My example is also used by Boss: 9v -> 3v bias
The other example is EHX DMM and uses -15v with bias of -9,96v (and their expander to 6.26v)

Both are not 'some people", or so I thought...
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Building your own stompbox / Re: BBD compander gain / output question
« Last post by Rob Strand on Today at 09:05:53 PM »
Here's how I just calculated it:

0dB  = 1mW into 600 ohms
0dB  corresponds to Vrms = sqrt(1e-3*600) = 775mV          ; a lot of people will know this number
Vav = (0.636/0.707)*Vrms = 0.9 * Vrms  = 698mV

Compressor formula:
gain  = sqrt(R1 R2 Ib /(2*R3 * Vin_av))
         = sqrt(10k * 20k 140e-6 / (2*20k * 0.698))
         = 1.0

I think this is correct now.
----------------
You just posted the same time as me.
Give me a sec to check your calcs.

I also screwed up my calculation. (now fixed)
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Building your own stompbox / Re: BBD compander gain / output question
« Last post by j_flanders on Today at 09:02:39 PM »
Thanks Rob, but I'm afraid I still don't understand.
Gain depends on input voltage.
I'm varying the input voltage.
How can the db always be 3 while the gain is different for each input voltage?

For example:

input: 10mV peak to peak:

Vav =  0.693*0.005 = 0.003465
gain= sqrt((10000*20000*0.000140)/(2*30000*0.003465)= 11,6 (+db around 20db ?)

output= 116mV

-------------------
input= 100mV peak to peak:

Vav =  0.693*0.05 = 0.03465
gain= sqrt((10000*20000*0.000140)/(2*30000*0.03465)= 3.6 (+db around 9,5 ?)

output= 3,6V
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Building your own stompbox / Re: BBD compander gain / output question
« Last post by Rob Strand on Today at 08:53:19 PM »
Quote
My first, side-question: The output is biased at 3V. Why wouldn't they bias around 1/2 which would be 4.5V?
Calculation for the output bias:
(1 + (10k + 10k)/30k)*1.8V = 3V
The examples in the datasheet are 3V because the supply is 6V.
Some people copy the datasheet for other voltages but the biasing is stuck on 3V and is non-optimal.
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Building your own stompbox / Re: BBD compander gain / output question
« Last post by Rob Strand on Today at 08:43:06 PM »
The formula for the gain uses Vin_av.

Vrms = 0.707 * Vpk
Vav   = 0.693 0.636 * Vpk

Vp = one sided peak (0 to the peak)

I should ask. How are you measuring the voltage? Peaks?



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Building your own stompbox / Re: HELP WITH A CHUNK SYSTEMS AGENT 00 MARK II
« Last post by DOND on Today at 08:06:15 PM »
great...will post pics, tony.....wish I knew what I was doing wrong/missing.i thought I was actually beginning to get the hang of this.when I layed the chips over each other and actually got LED2 (assuming its the clipping diode) to work and some action out of the pots I was getting close,but you guys know better than me.will post the pics of everything as soon as I can...once again,thanks for the input
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