Leestereo's Restoration/Upgrade of a Sansui AU-719

[Leestereo]

A little while ago (Fall, 2020), AK member @Phineas69 brought in a very nice AU-719 for a complete restoration and upgrade.

PART 1: HEAD AMP CIRCUIT BOARD (F-3095)

The F-3095 board contains the Head (Phono) Amplifier for moving magnet cartridges.

Signal Path Components:
In each channel, the output high-pass filter consists of a pair of polarized 47µF/35V capacitors in series (C27/C29 and C28/C30). Each capacitor pair was replaced with a single 6.8µF WIMA MKS4 capacitor. The film by-pass capacitors (C31 and C32) for the output capacitors which were 0.22µF polyester film types, were upgraded to 0.47µF polypropylene film types. The 3.3nF polyester capacitors (C33/C34) in the output low-pass filter were replaced with 180pF C0G types (to minimize the phase shift within the passband). In the RIAA equalization, the 1.5kohm carbon film resistors at R31/R32 were replaced with 1.0kohm metal film types (improves the 3180µs correction).

Non-signal Path Components
The F-3095 board contains two 22pF "Black Flag" polystyrene film capacitors at C03 and C04. These failure-prone types were replaced with C0G capacitors of the same capacity. The capacitors, C07 and C08, which shunt the zener diodes, ZD01 and ZD02 (respectively) were originally rated at 47µF/16V and their replacements were low ESR types rated at 100µF/35V. The 13V zeners (ZD01, ZD02) were also replaced with low noise, precision (2%) BZX55B13 types. The "speed-up" capacitors at C23/C24 were upgraded to 1uF stacked film types and the now superfluous film by-pass capacitors at C25/C26 were removed. There are 8 local decoupling capacitors (C35-C42)on the F-3055 board, arranged in 4 pairs; each pair consisting of a 100µF/35V capacitor in parallel with a 47µF/50V capacitor. Each pair was replaced with a single low ESR capacitor rated at 330uF/50V.

 

[GrisPato]

Nice to see a new restoration from you with all the extra information!
This one caught my eye immediately.

the output high-pass filter consists of a pair of polarized 47µF/35V capacitors in series (C27/C29 and C28/C30). Each capacitor pair was replaced with a single 6.8µF WIMA MKS4 capacitor.

I would love to understand how you go from 22-23uF electrolytic to 6,8 uF film.
I know you have good arguments because I remember you explained ones something similar like this, I just can not find that thread anymore.
Will be following this with interest!

 

[Hyperion]

I would love to understand how you go from 22-23uF electrolytic to 6,8 uF film.

I am guessing Ben will have adjusted the capacitance required in those positions to flatten or otherwise improve the RIAA response? I am also guessing that the much lower ESR of a film capacitor is playing a part in the quite dramatic value reduction.

 

[GrisPato]

Yes, I am sure about that to. Just want to know how he does that .

 

[Leestereo]

...I would love to understand how you go from 22-23uF electrolytic to 6,8 uF film.
I know you have good arguments because I remember you explained ones something similar like this, I just can not find that thread anymore...Will be following this with interest!

The output high-pass filter for the phono stage is a simple 1st order RC one. In the stock configuration (for the left channel), the "C" is ~24µF (C27 + C29 + C31), the "R" is ~32kohm (R55 in parallel with 47kohm). The resulting F3 of the stock configuration is 0.2Hz and with a 6.8µF film capacitor replacement, the F3 is 0.7Hz. Since the filter F3 must be at least 2 octaves below the lower limit of the desired passband to have a flat response, i.e., ≤5Hz for a lower limit of 20Hz, a 6.8µF capacitor is more than adequate in this case. Indeed, a 1µF film capacitor (or a 10µF electrolytic capacitor) is sufficient for achieve a F3 of 5Hz. So why use a 6.8µF film capacitor? The larger film capacitor also minimizes the phase shift induced by the filter; with a 1µF capacitor, the phase shift is 14°, with the 6.8µF capacitor, the phase shift is only 2.1°. Although there is much debate as to whether phase shift is audible, since the F-3095 board can accommodate a 6.8µF film capacitor, it was deemed a worthwhile upgrade. For comparison, in the AU-717 phono stage output filter, Sansui uses 2x 10µF electrolytic capacitors (with 37.5kohm resistance) for a F3 of 1Hz and a phase shift of 2.8°; in the AU-717 restoration a 4.7µF film capacitor was installed.

 

[GrisPato]

Thanks for this explanation. Don't get the 47k yet that should be parallel to R55 100k. But I think it's time for me to start to put some more time in studying this filter configurations.

The 3.3nF polyester capacitors (C33/C34) in the output low-pass filter were replaced with 180pF C0G types (to minimize the phase shift within the passband). In the RIAA equalization, the 1.5kohm carbon film resistors at R31/R32 were replaced with 1.0kohm metal film types (improves the 3180µs correction).

This to sounds like a worth while upgrade but don't understand how you get to those values.
So study is necessary.

 

[GrisPato]

I was wondering. Did Sansui use larger capacitance values with electrolytic capacitors to lower the distortion created by them on this high pass filters? And if so wouldn't be replacing them with film be the way to go because of proven and undisputed benefit of using a film capacitor ( that will fit) instead of a bi-polar electrolytic?
Or is the mentioned fase shift the limiting factor?

 

[Leestereo]

...Don't get the 47k yet that should be parallel to R55 100k...

So you want to know all my secrets?...The 47kohm is the input put impedance of the AU-719 line stage; the phono output is connected to the S01 Selector switch, same as other inputs.

 

[GrisPato]

So you want to know all my secrets?...The 47kohm is the input put impedance of the AU-719 line stage; the phono output is connected to the S01 Selector switch, same as other inputs.

Yes I would want to.
Funny because I wanted to say that in my post "don't want to know all your secrets".
I was thinking about input impedance but we not sure enough.
Thanks for sharing your secrets!

 

[Leestereo]

...Did Sansui use larger capacitance values with electrolytic capacitors to lower the distortion created by them on this high pass filters?...

Yes, when using electrolytic capacitors in a filter, its value should be ≥10x that of the film capacitor needed to achieve the target F3 to minimize distortion from the electrolytic capacitor within the passband. Below are distortion measurements (%THD) with a 47µF electrolytic capacitor, a pair of 470µF electrolytic capacitors (including a Silmic II) and 30µF film capacitors (from http://www.diyaudio.com/forums/parts/192811-capacitor-measurement.html#post2643470). As can be seen, by using the 470µF electrolytic capacitor, the distortion seen with the 47µF electrolytic capacitor is eliminated; the distortion with a 30µF film capacitor is also shown for comparison.

 

[ignoramustot]

 

[GrisPato]

each pair consisting of a 100µF/35V capacitor in parallel with a 47µF/50V capacitor. Each pair was replaced with a single low ESR capacitor rated at 330uF/50V.

This is interesting to. I was thinking if it would be a little upgrade to use the vacant place from the 47 uF to put in a 220-470 nF film in.
Then I checked the schematic of the AU-819 ( or AU-919) and found Sansui did this, 330uF electrolytic with a 220 nF film.
The AU-719 being the "cheaper" international replacement of the AU-819.
Would you consider this a worthwhile "upgrade"?

 

[Leestereo]

...I was thinking if it would be a little upgrade to use the vacant place from the 47 uF to put in a 220-470 nF film in.
Then I checked the schematic of the AU-819 ( or AU-919) and found Sansui did this, 330uF electrolytic with a 220 nF film...

Generally, the installation of a film by-pass may help with shunting higher frequency noise to ground; its inclusion (provided that it is physically small) will not do any harm. In this particular case, since the power to the phono stage is already from a regulated supply (F-3101) and the modern replacement low-ESR capacitors have much better high-frequency performance (after all they are intended for use in switching power supplies), than their vintage counterparts, IMO, a film by-pass is a "nice-to-have" rather than a "need-to-have" item.

 

[Leestereo]

PART 2: CONTROL AMP CIRCUIT BOARD (F-3098)

Signal Path Components:
The capacitors (C03, C04) for the "Loudness" function were 22nF polyester film types and these were upgraded to same capacity polypropylene film types. Similarly, the capacitors (C05,C06) in the high pass filter for the differential input stage were upgraded to 0.47µF polypropylene types. The garden-variety ceramic capacitors (C07, C08) in the input low-pass filter were upgraded to C0G types. The high-pass filter capacitors (C11, C12,), and their film by-passes (C09, C10), for tone control output were upgraded to 10µF bi-polar types and 0.1µF polypropylene film types, respectively. The feedback phase compensation capacitors (C13, C14) were upgraded to C0G types. The paired high-pass filter capacitors (C17+C19, C18+C20) were replaced with single 4.7µF polyester film types. Consequently, the 0.047µF film by-passes (C21, C22) for the paired capacitors were no longer necessary and were removed. The polyester capacitors (C23, C24) for the Subsonic Filter were upgraded to polypropylene types. The feedback resistors at R19, R20 were upgraded to metal film types.

Non-signal Path Components:
The local decoupling capacitors (C25-C28) were upgraded to 100µF low ESR types.

 

[ghamilton]

Can you tell us what caps were used in C25-28 ? They look like Panasonic FR or FM.

 

[Leestereo]

Can you tell us what caps were used in C25-28 ? They look like Panasonic FR or FM.

Panasonic FM.

 

[ghamilton]

With all the little improvements to this 719 I bet its close in performance to the 819.

 

[Leestereo]

PART 3: POWER SUPPLY & PROTECTOR CIRCUIT BOARD (F-3101)

The F-3101 board contains the regulated ±32VDC power supplies for the Head Amp and the Control Amp boards as well as the protection circuit for DC offset/over-current faults. A total of 18 original components were replaced/upgraded on this board. The initial DC filtering capacitors (C607/C608) were upgraded to 1000µF/63V low ESR types. The ZD602 zener diode was replaced with a precision (2%), low-noise variant and its noise-filter (C609) was replaced with a 220µF/25V low ESR type. The 1µF electrolytic capacitors at C612 and C613 were replaced with WIMA stacked film types. The final filtering capacitors (C614/C615) were upgraded to 330µF/50V low ESR types. The 68ohm 0.5W voltage dropping resistors at R634 and R635, which run hot (slight board discoloration), were upgraded to 68ohm/2W metal film types. In the protection circuit, the DC filtering capacitor (C601) was replaced with a 100µF/63V low ESR type. The capacitors at C603, C605, and C606 were replaced with low ESR types. The bipolar capacitors (C17/C18) were upgraded with 105°C variants. The ZD601 zener diode was replaced with a low noise/2% precision type. Although not shown in the pictures, the stock OMRON output relay was replaced with a drop-in Honeywell equivalent.

 

[Leestereo]

PART 4: DRIVER CIRCUIT BOARDS (F-2926 & F-2927)

The F-2926 and the F-2927 boards contain the Driver Stages for left and right channels, respectively; the circuitry is identical for both boards, only the component layout is different. The local decoupling capacitors (C16 and C17) were replaced with 1000µF/63V low-ESR types. Note that these large capacitors were attached to the board with glue during manufacturing and that it is best to remove all traces of the glue residue prior to the installation of the replacement capacitors since the aged glue is known to be corrosive. The 27V zener diode (ZD01) was replaced with a precision (2%), low-noise variant and its noise-filter (C01) was replaced with a 120µF/50V low-ESR type. Similarly, the 13V zener diode (ZD02) was replaced with 2%, low-noise variant and its noise-filter (C09) was replaced with a 100µF/35V low-ESR type. The noise filters for the MV103 triple-diodes consisted of electrolytic capacitors (C05 and C08), each with a polyester film by-pass (C06 and C07); these were replaced with 4.7µF WIMA stacked-film capacitors. Similarly, the Vbe multiplier by-pass capacitor consisted of an electrolytic type (C14) with a film by-pass (C15), and these were replaced with a single 1µF film capacitor. The signal path capacitors at C19 and C20 were upgraded to C0G types. The 1µF electrolytic capacitor (C24) in the Detection Circuit was upgraded to a film type. The feedback resistor (R39) was upgraded to metal film type. The trimmers for the DC offset and bias adjustments (VR01 and VR02, respectively) were upgraded to high-stability cermet types.

 

[ghamilton]

Are those black caps Panasonic FR?