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Chuck D. Bones

Chuck D. Bones

Circuit Wizard
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?
 
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?
 
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
Chuck D. Bones

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

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...
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See the open loop frequency response on page 4 of the datasheet
 
I generally find the biggest difference between Rats comes down to the tolerance of the 2.2uf cap connected to the 47ohm resistor.

If the cap is exactly 2.2uf then the frequency of the gain being applied is 1540hz . But, for example, if the cap is 2uf then the gain frequency is now 1694hz. Etc. Enough for an audible difference in tone.
 
Aluminum caps have loose initial tolerance and tend to run on the high side. I just pulled a 2.2uF aluminum cap out of a Rat and it measured 2.3uF. I'm putting a tantalum in its place that measures 2.18uF.

When analyzing the feedback network, we can't assume that the opamp has unlimited gain and bandwidth. The closed-loop gain depends on how the opamp's gain & bandwidth interacts with the feedback network. In the Rat, the loop gain drops below 0dB well below 1.5KHz.
 
I never bonded much with the standard Rat circuit. I liked it…played it some but never held onto one. I would like to have a BETTER Rat. Lately I’ve been wondering if there’s a hot rodded Rat or Rat variance on here that I may like. Maybe something like the Dingo, Frajeelay or Pandoras Box. Any suggestions?
 
I'm much the same. The Rat sounds very vanilla to me. I love adding a bass pot - that really helped. But still it's not the most engaging, complex sound IMO. I probably play the wrong kinda music for it. I have built some for friends with original round metal case LM308s and they love 'em but they do little for me.

The Expandora is a heap of fun though. The "forbidden" setting is more my kinda thing. If I ever build another one I'll just hardwire it to that setting.
 
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