[jeffreybehr]

Have done a bit more--
1. Higher-quality Cardas bindingposts...

...these being CCGRs which means copper nut/copper body/gold-plated nut/rhodium-plated body;

2. Added more SoundCoat to the bottom...

(I SuperGlued the Herbies' Tall Tenderfoots to the cover, but that didn't work, so they're again loose.

3. Replaced the four-per-amp 470pF RFI-filtercaps around the driver transistors with Rel-Cap RTEs...

...the little black rectangles in the corners.  The double-quad of o'drops flanking the powertransformer will be coming off.

4. Applied more SoundCoat to the inside and outside of the rear panel...


Have replaced a few other O-drops and will end up with only 1 very small pair plus the bypasses on the frontend-supply's 1st-pole caps in each amp.

Will be replacing the RCAs with Furutech FP-901Gs but only after I receive the final input-filter caps and resistor.  I'm not looking forward to that work, as there's not much room down there and the rearpanel looks as if it doesn't move to the the rear very far.

Am now scheming on a continuing and even-better improvement of the output-stage powersupply, with I-hope-double the amount of BlackGate-N 680/65s as DNA caps and, eventually, higher-quality initial-PS caps.

The amps continue to sound supergood with the V'steens, and I expect they'll get better as all these new caps, etc., break-in.

[jeffreybehr]

Got the new input-filter caps and resistor installed.  I chose equal values (0.01uF) of SoniCap Platinums (for clarity, cleanliness, detail) and Jupiter HTs (for wholeness and slight warmth).  With the initial PRP-brand 120K series resistor, the high-pass-filter point (HPFP) is 58Hz.  Installed them inside the amp.


The 120K resistor reduces the amp-system Voltage gain by 18.7dB which gets my preamp's level control close to as high as I want it.  I might order and install 150K resistors for a little less gain and which will move the HPFP to about 48Hz.

[tdinut]

Jeffrey,

It is amazing what you are able to do with these amps. Thanks for sharing!

Joe

[jeffreybehr]

Jeffrey, It is amazing what you are able to do with these amps. Thanks for sharing!
Joe

TY, and YW.  There's more to come, and I'll continue posting pics and comments.  These amps are naturals for my kind of parts-replacement projects--there's lots of space within, the circuitboard is strong and the traces fairly large, and the bottom cover comes off.  All that makes it fairly easy.

[Rclark]

That does look like a lot of fun. I hope to be able to tinker the same way soon, beginning with my next amps. The only hands on I've had so far was building crossovers.

[timind]

When do we get to hear about the sonic improvements? I did some upgrading on an older Harman Kardon amp and believe I heard significant improvements. Let us know your results, curious minds want to know.

[NuTreez]

So when all was completed to date: How much did the upgrades cost and can a relatively competent audio eng perform the mods? Does C-J offer the mods when sent the amps?
Thanks for the comprehensive step by step!
T

[jeffreybehr]

So when all was completed to date: How much did the upgrades cost and can a relatively competent audio eng perform the mods? Does C-J offer the mods when sent the amps?
Thanks for the comprehensive step by step!
T

I've not thought about it so I have no idea what the total cost so far is.  Probably it's not as much as one might think since the excellent BlackGate 65V. caps I'm using in the DNA positions are only $13 each.

Certainly an audio eng(ineer) could do what I've done; I'm not even a trained technician, I'm merely an audio hobbyist.

I can't imagine c-j doing the custom stuff I've done, but independent audio-equipment techs certainly could.  What I've done is really simple stuff--literally removing the medium-quality parts and installing HIGH-quality parts.

I'll continue to post my changes.

[jeffreybehr]

When do we get to hear about the sonic improvements? I did some upgrading on an older Harman Kardon amp and believe I heard significant improvements. Let us know your results, curious minds want to know.

I am NOT a quick-hearing-of-tiny-differences golden-eared audiofile; sometimes I think I'm more a tin-eared audiofool...and I'm definitely NOT an audiofile who describes sonic changes with BIG words...you know the ones...'black and white'...'huge'...and my favorite, 'totally different'.  I believe that all these changes are for the better and that eventually I'll perceive an audible improvement in my system.  Sorry I can't be more specific.

[Rclark]

There might not be such a thing as "golden ear" anyway. Might all just be marketing for gear companies. There is either you can hear so high or you can't.


 IMO, after everything I've seen as a noob, there should be a reality series called "American Greed: Audiophiles" and show just how much stuff gets bandied about as legit technology is nothing but a SCAM SCAM SCAM.

[ptmconsulting]

I'm especially curious why you removed the transformer coupled input and replaced it with a cap and resistor? Was that to get better synergy with your preamp?

I know there is a limitation on matching impedance pre to amp on the DNA 500 (which I own). Is it the same for the 750?

[jeffreybehr]

I'm especially curious why you removed the transformer coupled input and replaced it with a cap and resistor? Was that to get better synergy with your preamp?  I know there is a limitation on matching impedance pre to amp on the DNA 500 (which I own). Is it the same for the 750?

I didn't remove the transformer--it's still there, on the right of the pic in post #21 and here in a pic of the 2nd amp...

...along with its multicolored wiring.  If it weren't, half the amp wouldn't work, since the transformer creates the opposite-polarity signal to drive the 'other' half of the amp.  I added the series resistor and pair of caps before the transformer's input. 

The goals of this RC network are to:
1. Increase the amp's input impedance so that it and the bass amp's input impedance, paralleled, is easier to drive and also to keep the high-pass filterpoint of the preamp's output-coupling caps below 2Hz;
2. Create a much higher resistive value for the input-filter caps to work with.  The amp's nominal 10K input impedance would have required a larger cap value to achieve a HPFP in the 50H - 60Hz range.  BTW, I've calculated, using signal generator and Voltage meter, etc., that my amp's input impedance is 16.7K-Ohms.  This determination was so accurate that when I calculated that 0.02uF and 120K (added) would create a 58Hz FP, and that's exactly (within 1Hz) what I got after installing those parts.
3. Reduce the poweramp system's Voltage gain so that I can use my preamp's volume control in the gain, rather than cut, range.  The 120K resistor cut poweramp gain by almost 19dB, but my preamp, a multichannel c-j MET1, still has maybe 15dB of gain left unused.  IOW, I still operate the preamp at average levels of -15dB to -25dB.
 
So the input transformer is still there, 'splitting the phase' or, more accurately, creating an negative-polarity duplicate of the positive-polarity signal.

And yes, that preamp-to-poweramp impedance 'matching' problem did exist, especially with a 67K load in parallel*, and is eliminated by adding the resistor.  You too can do that with your '500.  Of course, the higher resistor value you choose, the more it drops Voltage, which requires higher Voltage drive from the preamp.  Passive 'preamps' need not apply, as the combination of low-gain poweramps driving relatively low-sensitivity speakers requires high-gain frontends.   

Another interesting (to me, at least) thing about this/these amps--altho they're fully differential/'balanced'/push-pull, the positive-and-negative-signals feature of a 'balanced' input using an XLR connector is not used.  Only the XLR's positive lead is used, joined to the RCA's positive input at the RCA's positive terminal.

* yielding an actual 13.4K-Ohms

[ptmconsulting]

I know you bypassed all the chips on the output board with Sonicaps. Here's a trick I learned from Bob Crump when he voiced the JC1. I did this on my DNA .5 when I upgraded it many years ago (using RelCap RT's which is what he used at the end of the rail on the JC1 to bypass the Nichicon's).

BC suggested not putting bypasses on all the output devices. Instead he suggested bypassing only the "inner" chips (closest to signal origination) and leaving the other half / output side chips alone. His feeing was that too much of a good thing was too much, and this was a nice balance.

Just a thought in case you find that things sound too open afterwards and need to tweak it.

[jeffreybehr]

Got and installed the final parts before I tackle the DNA-cap expansion, which will at least triple the 680uF-times-8-per amp.

The top shows:
1. The new speakercable, composed of 1 strand of Neotec UPOCC 18g. copper plus 2 strands of 20g. silver in Teflon airtubes, per pole;
2. Just how few capacitors are left on the top of the board; and
3. The new Furutech FI-10(R) IEC power inlet, this time a screw-type instead of solder-type.


The bottom. 

The only orangedrop bypass caps are on the 1st pole of the frontend PS.  The 4 medium-sized white caps are the output-section fuse bypasses, now MultiCap RTXs.

[Rclark]

What sort of camera are you using to take these excellent close up shots?

[timind]

What sort of camera are you using to take these excellent close up shots?

Good question! The pics are wonderful.

[jeffreybehr]

TY, gentlemen.   

I use a Canon 5D and, virtually always for this kind of stuff, a Canon 24-105/F4/IS/L lens.  Sometimes I need to add a 12mm extension tube for shots closer than, say, a foot from the lens.

I have several daylight-balanced (c. 5K color temp.) flourescent floodlites that I use for working on and fotografing the amps, and I positioned about 4 for these shots.  I also use a Canon 420EX flash rotated to bounce off a wall/ceiling.  Almost all of this stuff is shot handheld--and the Image-Stabilized lens REALLY helps eliminate camera shake--altho sometimes I use a 'monopod', that being a stick about 4' long on which I rest the camera.  I use Photoshop CS 5 (PS 12) to develop the images.

One 'secret'--NEVER position the camera squarely above or in front of a reflective surface you're fotografing...unless you want a HUGE, bright reflection in the images.  For these 2 shots, I positioned the camera above the rear panel--the front edge of one sandal is barely visible in both.  Cropping in PS using the Perspective control eliminates the the nonsquareness of the square that occurs because the rear of the rectangle is farther away from the camera than the front.

Here's 'The Lab' and 'foto studio' with one of my little Marantz MA-22s serving as the victim.

Visible are 3 work/fotofloods plus my reading lamp moved into position.

[Levi]

Good job.  Thanks for sharing!

[djbnh]

I like in your photo that I can see your "fan" is off the grid - LOL. Forgetting that little joke, thanks for sharing and good luck; nice skill set you possess.

[K Shep]

TY, gentlemen.   


Visible are 3 work/fotofloods plus my reading lamp moved into position.

Looks like a Conrad Johnson CT5 in the rack...NICE!