Yamaha P2100 recap and upgrade: suggestions?

[pradigmshift]

Hi all,

I'm going to recap my Yamaha P2100 (schematics here, and added as zip). It sounds very good as-is to me, nice '77 design.

In determining 'things-to-do' besides recap I was comparing it to the Yamaha P2150 (added file) and other Yamaha power amps (like this website does). I am wondering if I can add to the amplifier board a 220 uF decoupling from B+ to E and from B- to E, with parallel diode like in the P2150, and the R+C combination to the heat sink?

Also, I don't understand how the power supply in the P2100 works for the input side (the 10.2 VAC), how can it be that the small rectifier's plus side is connected to the B- supply?

Things I will do are:

• increase bias for lowest distortion
• add speaker protection kit Velleman K4700
• new bridge rectifier (old one is only 5 A)
• bypass cap for the large cans
• metal film in signal path
• MOX 10W for emitter resistors
• new input diff. pair transistors A970
• new binding posts

Any suggestions appreciated.

[pradigmshift]

Update: just did the recap and

• increased bias from 33 mV to 52 mV (works well, but still have to measure best value)
• added speaker protection kit Velleman K4700
• new bridge rectifier (old one is only 5 A)
• new binding posts
• bypassed volume control
• bypassed on/off switch

See the pictures below (red is adjustments):

Right channel diff/driver board:


Right channel output board:


Sounds much better! Note: there is an error in the schematic, there is a 0.01 uF missing between TR14 and TR15 (it's drawn correct on the left channel, not shown).

To do:

• bypass cap for the large cans
• metal film in signal path
• MOX 10W for emitter resistors
• new input diff. pair transistors A970
• replace driver transistor with MJE15030/15031

Any suggestions? Not sure about what can be changed to the circuitry for better performance, but would like to know.

[sakis]

by passing rails to a locan ground could be more bad than good and if you added clamp diodes 1N4007 is not enough /.... try something like 1N5408

[pradigmshift]

Thank you, 1N5408 added to parts list. Yes, the adjustments I show in the pictures I have already done, the amp does (still) work.

You are saying that, depending on topology, local psu bypass may work or not. I was assuming that local psu bypass is always good and thinking that it gives similar effect in the P2100 as it does in the P2150 (schematic added in first post)?

What else could I do to improve the design?

[sakis]

yes but is grounded on the heatsink ???? ground rooting is a major issue in amplification

[pradigmshift]

Agreed, grounding scheme is very important. That's why I took the P2150 schematic as starting point. 220 uF is grounded to the boards earth, but the series combination 0.22 uF + 10 Ohms is grounded to the heatsink. I presume this is to prevent oscillation. There is no noise coming off the loudspeakers, almost completely silent.