Lar/Mar and PI filtering

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Elad E
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Lar/Mar and PI filtering

Post by Elad E » Tue Oct 17, 2017 6:42 pm

my '68 spec Superlead build felt overly mushy and fuzzy with the Lar/Mar PPIMV turned down to less than noon.
I noticed that ramping PI filtering from 50uF to 100uF got rid of the mushiness when running the PPIMV low.

Has anyone else found the need for higher PI filtering with a PPIMV?

Tazin
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Re: Lar/Mar and PI filtering

Post by Tazin » Wed Oct 18, 2017 5:01 pm

Well, 50uF for the PI filtering is not the norm for the split-cathode style Super Leads...More like 80uF or 100uF. The earlier (67-68) share-cathode style Super Leads ran 48uF in that spot.

Elad E
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Re: Lar/Mar and PI filtering

Post by Elad E » Thu Oct 19, 2017 7:49 am

I’m pretty sure 32+16 was common on split cathode amps during ‘68 but regardless, my amp went through many circuit variations including a ‘68 Superbass spec (shared cathode, 56/250 tone stack, .1 PI coupling caps) and was still fuzzy and mushy with 50uF PI filtering except when the PPIMV was set higher than 12 oclock.

Maybe it’s the Dagnall style iron that calls for higher PI filtering to keep the low end together.

Tazin
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Re: Lar/Mar and PI filtering

Post by Tazin » Thu Oct 19, 2017 4:41 pm

The fussy part is probably from the nfb circuit since the Lar/Mar PPIMV messes with it at lower settings. That's why the Presence control doesn't seem to do much when the PPIMV is at lower settings.

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neikeel
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Re: Lar/Mar and PI filtering

Post by neikeel » Fri Oct 20, 2017 9:26 am

Tazin wrote:
Thu Oct 19, 2017 4:41 pm
The fussy part is probably from the nfb circuit since the Lar/Mar PPIMV messes with it at lower settings. That's why the Presence control doesn't seem to do much when the PPIMV is at lower settings.
exactly :rock:
Neil

Elad E
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Re: Lar/Mar and PI filtering

Post by Elad E » Fri Oct 20, 2017 5:00 pm

In this case, would a Rich mod PPIMV be less prone to the same NFB deficiencies of the LarMar PPIMV?

Tazin
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Re: Lar/Mar and PI filtering

Post by Tazin » Fri Oct 20, 2017 8:07 pm

Both PPIMV's are similar in design and overall function; therefore, I would not expect to hear a big difference.

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