Leestereo's Restoration and Upgrade of a Sansui AU-777A

[Leestereo]

Please correct me if I'm wrong but that seems only partly right to me. Read-on, but essentially a connected speaker is much more efficient path to ground than the 150 Ohm resistor. I.e. the resistor may (*) enable defeating the thump when using the speaker-OFF start-up method but won't help much (~10%) for Speaker A or B start-up position.....

Admittedly, when I first noticed the 150ohm resistors in a Sansui capacitor-coupled amplifier schematic, I did question if such an arrangement could effectively mitigate the "turn-on" thump, since as you point out, speakers are typically rated at 4-8ohms. But as they say: "the proof is in the pudding" and the capacitor-coupled Sansui amplifiers that have been on my bench (555A and 777A) have not exhibited any obvious "turn-on" thump (except for the example noted below). In comparison, the capacitor-coupled Marantz 1060 amplifier does produce an obvious (audible and visual) "turn-on" thump; Marantz designs do not have resistors that connect the output capacitors to ground. The effectiveness of the 150ohm resistors may be due to the fact that they are in fact in parallel with a complex RLC circuit when a speaker is connected.

*However, be aware that such thumps are sometimes due to other start-up transients upstream, and non-transient DC offset can even cause this too. I.e. if switching speakers to OFF during start-up solves your thump problem (even when later switching to speaker) in a model without the resistor, it may not be output cap charge-up related and addition of the resistor may not solve the issue.

An example that illustrates this point is a 555A that exhibited a loud "turn-on" thump; the cause was a broken trace in the regulated supply to the power amplifier front-end (see Repair and Upgrade of a Previously Recapped Sansui AU-555A for additional details.

 

[smurfer77]

Admittedly, when I first noticed the 150ohm resistors in a Sansui capacitor-coupled amplifier schematic, I did question if such an arrangement could effectively mitigate the "turn-on" thump, since as you point out, speakers are typically rated at 4-8ohms. But as they say: "the proof is in the pudding" and the capacitor-coupled Sansui amplifiers that have been on my bench (555A and 777A) have not exhibited any obvious "turn-on" thump (except for the example noted below). In comparison, the capacitor-coupled Marantz 1060 amplifier does produce an obvious (audible and visual) "turn-on" thump; Marantz designs do not have resistors that connect the output capacitors to ground. The effectiveness of the 150ohm resistors may be due to the fact that they are in fact in parallel with a complex RLC circuit when a speaker is connected.

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An example that illustrates this point is a 555A that exhibited a loud "turn-on" thump; the cause was a broken trace in the regulated supply to the power amplifier front-end (see Repair and Upgrade of a Previously Recapped Sansui AU-555A for additional details.

The RLC impedence does help the argument, but I wouldn't credit it with have significant enough magnitude of impedance over relevant frequency range to enabling shunting even close to majority of the thump via 150 Ohm. Well, maybe I am wrong since we need to be careful, considering the frequency spectrum resultant from the thump.

But lots of models with similar resistor thump nicely. My Sansui 350 for example and several others I own have similarly place resistors. To be fair they used 680 Oms in the 350, which obviously won't 'shunt' as much as 150 Ohm when in parallel with a speaker, but the choice of such a large value resistors makes it quite clear that competing with speaker load for shunting was not the intention.

EDIT: I just clipped out the 150 Ohm resistors in my AU-777A. No thump at turn on with the 150 Ohm resistors removed from circuit. I.e. The 150 Ohm is not responsible for defeating turn on thump

 

[Leestereo]

...EDIT: I just clipped out the 150 Ohm resistors in my AU-777A. No thump at turn on with the 150 Ohm resistors removed from circuit.

That's interesting, not what I would have expected; was there a residual charge on the output capacitors?

 

[smurfer77]

That's interesting, not what I would have expected; was there a residual charge on the output capacitors?

Same result with cap confirmed discharged. No thump.

 

[Hyperion]

That's what I would have said was happening here - namely, just the fact that you have a resistor connected from the speaker + output to ground, will pull down the OP capacitor charge voltage that collects there (and fools so many noobs measuring DC offset), and thus should significantly alleviate the switch on thump. And also that the value of the resistor doesn't need to be close to the impedance of the speaker for it to work in this way.

But if this is true, it suggests that with no resistor present, you will always get 'thumps' - unless perhaps the power rails come up very slowly?

 

[smurfer77]

That's what I would have said was happening here - namely, just the fact that you have a resistor connected from the speaker + output to ground, will pull down the OP capacitor charge voltage that collects there (and fools so many noobs measuring DC offset), and thus should significantly alleviate the switch on thump. And also that the value of the resistor doesn't need to be close to the impedance of the speaker for it to work in this way.

But if this is true, it suggests that with no resistor present, you will always get 'thumps' - unless perhaps the power rails come up very slowly?

John, the speaker is also connected from +output to ground. If the parallel resistor is not sufficiently small compared to speaker, it is the speaker that mostly pulls the cap down with most current going through the speaker, no?

I think the resisitor purpose is to provide path to ground in speaker OFF mode, maybe to protect headphones or to allow then non-thump switching to A or B speaker. Either that or Sansui wanted to ensure the output always saw a load even with speakers disconnected

 

[RobinTW]

Part 3: POWER AND PROTECTOR BLOCK (F-1153)

The F-1153 board contains the regulated power supplies as well as the protection circuit.

The stock rectifiers, D002 and D003, were replaced with 2A soft recovery types. The voltage dropping resistor at R007 was a 330ohm/3W cement type and was replaced with a 330ohm/ 5W low TCR wirewound. The 220µF/50V capacitor at C007 was replaced with a 470µF/63V low ESR type. Similarly, the 220µF/75V (C006), 470µF/35V (C005) and 220µF/50V (C003) capacitors were replaced with low ESR types rated at 330µF/80V, 1000µF/50V, and 680µF/50V, respectively. The 0.5W resistors: 1.5kohm (R001), 3.9kohm (R002) and 1.0kohm (R003) were upgraded to 2W metal film types. Similarly, the 1.0kohm/0.25W resistor at R004 was upgraded to a 0.5W metal film type. The 220ohm/5W cement resistors at R006 and R007 were upgraded to high stability vitreous enameled wirewound types rated at 7W (note that these were installed with their bodies well away from the board).

Question: How do I make adjustments if I want to replace VR001 of board F-1153. I noticed you didn't replace it in your restoration. Why?

 

[Hyperion]

John, the speaker is also connected from +output to ground. If the parallel resistor is not sufficiently small compared to speaker, it is the speaker that mostly pulls the cap down with most current going through the speaker, no?

I think the resistor purpose is to provide path to ground in speaker OFF mode, maybe to protect headphones or to allow then non-thump switching to A or B speaker. Either that or Sansui wanted to ensure the output always saw a load even with speakers disconnected

Yep of course understood, I was incorrectly thinking there was a speaker relay involved.

 

[stereofun]

Yep of course understood, I was incorrectly thinking there was a speaker relay involved.

.....speaker relay - what is a speaker relay ?....is that something on modern amps from the 1970'ties ?

 

[Bhuvaneshwar]

A little while ago (last summer), a very nice AU-777A was on the workbench for a complete restoration and upgrade.

Part 1: HEAD PRE-AMP BLOCK (F-1194)

The F-1194 board includes the phono stage (600-series components) and the line level stage (700-series components).

A. Phono Stage
The stock capacitors (C601/C602) in the high-pass filter for the phono input were 1.5µF polarized electrolytic types; these were upgraded to 1.0µF polyester film types. The emitter bypass capacitors at C605/C606 were originally 33µF/10V and were replaced with 47µF Nichicon KZ types; the increased capacity of the replacements extends their effectiveness to 20Hz. Similarly, the 10µF emitter bypass capacitors at C613/C614 were replaced with 100µF Nichicon FG types. The C607/C608 capacitors in the feedback high-pass filter were 33µF polarized types and these were replaced with 100µF Nichicon ES bipolar types. The stock phono stage output capacitors (C611/C612) were originally 10µF polarized electrolytic types and were replaced with 22µF Nichicon ES bipolar types. The failure-prone 2SC458 transistors (TR601-TR604) were replaced with KSC945C(G) transistors (hFE matched pairs).

The capacitors for the RIAA correction, C615-C618, were originally 10% polyester film types and these were upgraded to 5% polypropylene types. The RIAA resistors at R625/626 were originally 22kohm 10% carbon film types and their replacements were 22.6kohm 1% metal film types. Similarly, the RIAA resistors at R627/628 were originally 270kohm 10% carbon film types and the replacements were 1% 267kohm metal film types. Note that changes in the resistor values are deliberate and improve the RIAA accuracy to -0.1dB/+0.2dB. Furthermore, although the stock component values can theoretically yield excellent RIAA accuracy, given the 10% tolerance of the stock components, that is not very likely.

B. Line Stage
The input high-pass filter capacitors (C721/C722), originally 0.22µF polyester film types, were upgraded to 0.47µF Panasonic polypropylene film types. All other stock polyester signal path capacitors (C731-C738) were upgraded to polypropylene film types of the same capacity. The coupling capacitors at C727/C728 were originally 10µF polarized electrolytic types and these were replaced with 22µF Nichicon ES bipolar types. The 1µF signal path capacitors at C729/C730 and C739/C740 were upgraded to stacked film types. The 47µF emitter bypass capacitors at C725/C726 were replaced with 100µF Nichicon FG polarized types; the increased capacity extends their effectiveness to below 20Hz. The failure-prone 2SC458 transistors (TR705-TR710) were replaced with KSC945C(G) transistors (hFE matched pairs). The local decoupling/filtering capacitor at C743 was originally 470µF/25V and was replaced with a 1200µF/35V low ESR type.

I am still in the learning stage so apologies in advance if the question seems stupid.

It is seen that while replacing the capacitors, you have changed the type of capacitor. Eg below
1. Polarized electrolytic replaced by polyester film types
2. Polyester film replaced by polypropylene
Polarzed caps replaced by bipolar and so on

Generally it is said that polypropylene are better than polyester then why you have opted for polyester capacitor. Is it due to the change in capacitance of a polypropylene with temperature?
Also different series of Nichicon have been used depending on the location.

It would be kind of you if you could come up with a basic writeup on the selection of the capacitors so that greenhorns like me can have clarity on the subject. I know there is a ton of information on the subject but it would be easy to understand vis-a-vis the circuit in question.

 

[Hyperion]

It is seen that while replacing the capacitors, you have changed the type of capacitor. Eg below
1. Polarized electrolytic replaced by polyester film types

Usually for electrolytic capacitor values up to about 10µF - this is considered an upgrade. (in audio equipment).

2. Polyester film replaced by polypropylene

Polypropylene is better than Polyester as a capacitor dielectric - so also considered an upgrade. (in audio equipment).

Polarzed caps replaced by bipolar and so on

Personally I am very cautious about doing this, but I have seen and understood the advantages. However, this does not mean it is Ok to do this on a large scale. Most commonly done when two 'back-to-back' polarised electrolytic capacitors are used (to create a bipolar capacitor), replacing these with a single, purpose made 'quality' bipolar capacitor can make a definite improvement. But also for special cases where a 'polar' is replaced with a 'bipolar' - however this is unusual and should only be done with the guidance of the experts on here.

Generally it is said that polypropylene are better than polyester then why you have opted for polyester capacitor.

Polyester IS better than electrolytic, but not better than Polypropylene.

Is it due to the change in capacitance of a polypropylene with temperature?

No.

Also different series of Nichicon have been used depending on the location.

Yes, this is quite normal and represents Leestereo's attention to detail in his restores. I am also a very strong believer in 'right capacitor, right place' - for example the use of low ESR capacitors for 'filtering' and in power supplies, rather than 'audio grade' types which don't necessarily have very low ESR, or high temperature rating for these locations. Manufacturers don't make the different ranges of electrolytic capacitors they supply for fun, or so you can give a technicolour 'look' to your restores. The ranges have different characteristics (with some overlap), most suited to their various uses in electronic equipment. Such as:- low ESR, low leakage, high temperature, audio grade, bipolar, and so on...

I am sure Leestereo will correct, clarify, or add any further comments he may have.

 

[Bhuvaneshwar]

Usually for electrolytic capacitor values up to about 10µF - this is considered an upgrade. (in audio equipment).
Polypropylene is better than Polyester as a capacitor dielectric - so also considered an upgrade. (in audio equipment).
Personally I am very cautious about doing this, but I have seen and understood the advantages. However, this does not mean it is Ok to do this on a large scale. Most commonly done when two 'back-to-back' polarised electrolytic capacitors are used (to create a bipolar capacitor), replacing these with a single, purpose made 'quality' bipolar capacitor can make a definite improvement. But also for special cases where a 'polar' is replaced with a 'bipolar' - however this is unusual and should only be done with the guidance of the experts on here.
Polyester IS better than electrolytic, but not better than Polypropylene.
No.
Yes, this is quite normal and represents Leestereo's attention to detail in his restores. I am also a very strong believer in 'right capacitor, right place' - for example the use of low ESR capacitors for 'filtering' and in power supplies, rather than 'audio grade' types which don't necessarily have very low ESR, or high temperature rating for these locations. Manufacturers don't make the different ranges of electrolytic capacitors they supply for fun, or so you can give a technicolour 'look' to your restores. The ranges have different characteristics (with some overlap), most suited to their various uses in electronic equipment. Such as:- low ESR, low leakage, high temperature, audio grade, bipolar, and so on...

I am sure Leestereo will correct, clarify, or add any further comments he may have.

Thanks @Hyperion. Quite informative.

One more clarification please.
Would it be okay to replace electrolytes with polypropylene instead of polyester and vice versa or the replacement (polypropylene or polyster cap) is place dependent w.r.t amp circuits.

Please share this aspect also

 

[Hyperion]

Would it be okay to replace electrolytes with polypropylene instead of polyester

This is the recommended change.

You can replace an electrolytic with a polyester capacitor, but even better is to use a polypropylene.

Note:
All these capacitor changes depend on having the available space to install them, film capacitor replacements for electrolytics are always bigger. If there is so little room that you have to use wire extensions to the leads for example, then it probably isn't a good idea to change the capacitor type, and you should keep to the original type. Also if the area around a capacitor to be replaced gets hot, you almost certainly wouldn't want a bulky/large replacement component in there, as this could restrict air flow and therefore cooling with detrimental effect. Finally, a large or heavy PCB mounted replacement component should have additional support provided by whatever means are available, the PCB tracks alone are unlikely to be sufficient.

 

[rasenthiran]

Hi Guys,

Are you saying that the start up "thump sound" is bad for the speakers? I am reffering to my Sansui AU 777 which has this thump sound every time i "ON" the amplifier. Can you please advise, this is a fully recapped unit. I have asked my technician before, according to him it is normal for this kind thump sound for vintage amplifier. What worries me is i am using expensive speaker connected to the AU 777.

 

[SansuiJavier]

My 777 also does it, now I have to put the speaker switch to "B" when I turn off the amplifier, and go to "A" when I use it again, waiting about 30-40 seconds after switching on. If I do not wait for that time and change the speaker input immediately after powering on, keep doing a "pop". I guess it's the time it takes for the amplifier to "stabilize" after the start-up ...

 

[RobinTW]

My AU-777 only moves the woofers slightly when it's turned on. The movement is inaudiable, so I never paid attention to that.

 

[Henry Morgan]

I just had some altec 14's restored, plugged in
The 777a and on power up my my full range woofer
Jumped 3 times.. Futher than I have ever seen a woofer move under power.. I turned off the turntable
And just turned the volume control up and down
On the amp . It still mad the woofer jump without
Any sound

 

[AnalogueBen]

A little while ago (last summer), a very nice AU-777A was on the workbench for a complete restoration and upgrade.

Hi Leestereo,
As one amongst many others who were keenly following this thread, I'm wondering if you've finished writing it up? Still very interested to hear about your listening experience!

 

[quadklipsh]

Id like to convey my tech to follow leestereo and hyperion in restoration of my 777D.
And am sooper interested in sonic improvements that Lee finds now in the restored amp.
Plz throw detailed account n the pre and post restoration sound quality.

 

[quadklipsh]

Have you guys been able to play the amp yet or is this still under maintenance