[AudioCats]

starting a thread to document some info and measurements.

used 727A from yahoo.jp auctions, via Buyee.


Transformer is 100v only. All external wiring are soldered to transformer posts, but only 100v winding magnet wires are available. (connecting to white, green, gray, brown).

temp measurements, with cover removed, after 1hr. ambient 25c.
large heatsink: 56c
outer red heatsink (on circuit card): 52c
inner red heatsink (on circuit card): 56C
Q17 (on base board, no heatsink): 49C
Q21 (on base board, no heatsink): 52C

voltages: (input voltage 100.5v)
+Vh: 360v
-Vh: -357v
-Vh2: -392v
+/-15v: at +/-15v (these are regulated, should be very close to target)

all four 220uf/400v Elna power capacitors have slightly bulged top. Not sure if this indicates the amp was very very hot at one time.

-Vh2 is done by adding another voltage (involves D13, D14, C22, C23) on top of -Vh.

 

[AudioCats]

Shunt volume control mod
the amp works fine. The sound is kind of thick and a little dull. The alps rk27 can be improved upon by converting to shunt-pot style connection.

727 has a "direct/variable" switch which allows by-passing the rk27, a very nice touch. This by-pass function will remain intact with the shunt-pot mod. You can use either the shunt-modded rk27, or set to "direct" and use an external preamp.

shunt-pot mod requires cutting 4 traces, and make 4 shorts. Having this pcb makes it easy to implement.

****** Selecting the series resistors:
https://www.head-fi.org/threads/sma...ier-modification-thread.766837/#post-11608320


the stock sound is like of thick so something more "light and liquid" will help ---> I will use Caddock TF020 ceramic chip resistors.

Traces cut.

Caddock TF020 added.

 

[AudioCats]

Non-global / global feedback (global-feedback = "spritzer" mod)

727's feedback sample point is at the end of the 3rd (voltage gain) stage, before the 4th (current boost) stage, it is considered as non-global.
717 has global feedback, sample point at the output (end of 4th stage). Many people have said that the "spritzer" (global feedback) mod is a must-do for the 727, as it will provide much better bass control.

I have added a switch to my kgss to allow toggling between global/non-global feedback style, and I prefer non-global feedback in the kgss. Not sure which style I might prefer in a 727, I will add the switches to the 727 circuit cards as well.

the main challenge is mounting the switches. I used a L shaped aluminum bracket and attached it to circuit card utilizing the two upper heatsink screws.

L bracket is done by modifying a small aluminum to220 heatsink. Extra long mounting screws are required, with plastic stand-offs to keep the aluminum bracket away from circuit card components.

Spacers are Mouser # srs-1-5mm, (5mm length)

to help secure the attachment points to the global feedback pads, two small holes were drilled through the PCB and a piece of copper wire added to form the anchor point.

Added 300k resistors and teflon insulated wires. The arrangement is a bit messy but should still work ok.

levers go toward "1" = "717" = global-feedback

impressions, feedback style, global vs non-global (717 style vs stock 727 ), driving sr007
similar to the impressions from my experiments done on my KGSS (which also have the feedback switchs), though to a lesser degree.
-- "717/global" mode offers more defined bass; the sound a bit more closed-in but also more focused. Has a slight "sheen" feel to it. To me the vocal is less emotionally involving.
-- "727/non-global" mode: slightly larger stage; more lay back; more smooth, vocal more involving, I can get absorbed into the song better; the bass control is not that great. for example, in Eva Cassidy's Night Bird (from the night bird album) there is a lot of low bass. In 727 mode the bass is a wooly mess at times; in 717 mode the bass definition is quite ok.
When driving sr007 I'd lean toward the global/spritzer-mod mode a little more, as it cleans up the bass, and the "sheen" is not that obvious. When using ESP950 (brighter than sr007) I'd pick the 727/non-global mode.

if you are debating whether to do the global feedback mod to your 727...... the main question to ask is "with my current phones, can I live with the bass from the stock 727", if the answer is yes then maybe just enjoy the music and leave the circuit cards alone.

 

[purk]

Thanks for document your findings.