Rodentia

[Chuck D. Bones]

I have been working on three Proco Rat pedals for a client. Also did some breadboarding. I have observed that the tone of a Rat is strongly dependent on the particular LM308 installed. These are all legit parts, but there is enough part-to-part variation to make a noticeable difference. Has anyone else observed this?

 

[jwin615]

Can't say I have tried enough back to back to notice. Think this more likely a variance in GBW from manufacturing altering the rolloff--
or drift/abuse over the years? I can only imagine that a lot of these chips have changed hands over a dozen times by now.
Or are we talking a mix of Moto, Harris and NatSemi?

 

[Chuck D. Bones]

I don't know if the brand matters. I have Mot & Nat'l. GBW is definitely one of the factors affecting tone.

 

[Aentons]

General Buffer Widget?

 

[Erik S]

General Buffer Widget?

Great Big White Washer

 

[jwin615]

Gain bandwidth product. which affects open loop freq response. This is what's responsible for the high frequency rolloff at high gain in a 308 rat.
Chuck touches on it some in the below thread

Post in thread 'LM308 and OP07 - the plot thickens'

Aug 10, 2023

It's not just about slew rate, although slew rate is definitely a factor in the Rat's tone.
The other factors to consider are:
1. Gain as a function of frequency aka open loop frequency response
2. Output impedance and drive capability
3. Recovery from saturation

The first one is characterized in the datasheets. The datasheets provide graphs showing typical values. With a 30pF compensation cap, the LM308 has around 52dB gain at 1KHz. With DIST dimed, the gain of a Rat peaks out around 60dB and 600Hz. Gain falls off above that due to the limited open loop gain. Not only does the...

• Chuck D. Bones

• 

See the open loop frequency response on page 4 of the datasheet