Sherwood S-7300 Wolverine conversion - diyAudio
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Old Yesterday, 03:22 PM   #1
jpc2001 is offline jpc2001  United States
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Default Sherwood S-7300 Wolverine conversion

I'm planning to modify an old Sherwood S7300 with new power amp and preamp sections. I've done this before.

The power amp will be a clone of ostripper's Wolverine with near-PPM THD and short circuit protection. It'll use an SSLR for DC speaker protection and muting.

There are no coupling caps in the power amp. This power amp will be permanently mated to its preamp; a coupling cap at the preamp input will maintain zero DC offset throughout. There are no electrolytics in the signal path and only 3 caps total in the signal path. All are small enough that you could use film types. I'll probably use c0g ceramics, they're small and cheap and measure near zero distortion.

The preamp is a single gain stage with non-inverting Baxandall tone controls. I'm planning to use a spare pair of OPA627s.

This Baxandall network isn't symmetrical, it has a gain of 3. That introduces a quirk: to set the tone controls to flat, the original pots should be turned to a 25K/75K position at about 3:00 o'clock or so. You could fix that by adding extra resistance in the tone network, though that would raise the noise floor. I'll probably just mark a new center position in pencil on the front panel.

There are some nice original features of this Sherwood that I'll preserve:
* It has good grounding discipline, there's a dedicated signal ground parallel to the signal path which carries no unnecessary current.
* The volume pot is only 50K, not 100K as in a lot of 70s gear, so that's less noisy.
* The physical layout of the power supply will allow the half-wave output currents to be kept far from small signals. (Well; the power supply is a little bit close to the tone board, it could use a piece of shielding. It's not horrifying.) There's room to build the whole output stage on the rear of the heatsinks, opposite the amp input and driver stages, for better isolation of those half-wave currents.
* Let us praise 39V rails; they make SOAR protection pretty easy even with a single output pair. Simulating with a 4R load, the modified amp can drive the output to over 30V peaks before distortion exceeds -100db as current limiting is just starting to kick in. That's over 110 wpc, if I did the math right. The power supply probably couldn't keep up with a 110wpc test tone, but you'd have a chance to hit musical peaks in that vicinity.

Schematics to follow.
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Old Yesterday, 03:56 PM   #2
jpc2001 is offline jpc2001  United States
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Default schematics

Here's the LTSpice deck; the two files ending in '.asy.txt' should be renamed to '.asy' instead so that spice can recognize them.

Plus, png's for the amp and preamp.

There are some inaccuracies here:
* The real amp will likely use 2SA1507 and complement for drivers, I just don't have spice models for those.
* "R41" in the protection circuit is not really a gigohm resistor, it's really a wire. That just fools spice into unmuting the muting circuit from the outset of simulation.
* The PC817D optoisolator and its associated voltage source and 680R, those aren't real. The real circuit will use a PVI5013R in this place, like this one.
Attached Images
File Type: png s7300_amp.png (127.3 KB, 20 views)
File Type: png s7300_pre.png (106.9 KB, 17 views)
Attached Files
File Type: asc amp_mod.asc (13.9 KB, 0 views)
File Type: asc new_s7300.asc (6.1 KB, 0 views)
File Type: asc preamp_mod.asc (4.5 KB, 0 views)
File Type: txt preamp_mod.asy.txt (540 Bytes, 0 views)
File Type: txt amp_mod.asy.txt (1.1 KB, 1 views)
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Old Yesterday, 04:19 PM   #3
jpc2001 is offline jpc2001  United States
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Here's the top level schematic, including the output stage.
Attached Images
File Type: png s7300_top.png (138.8 KB, 16 views)
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