Sansui AU-505 Phono Stage schematic help - Capacitance

[its_chicken!]

Hello everyone,

I'd love some help in determining the capacitance of the phono stage in my AU-505 amplifier. I have attached a pic of the schematic. Any help is much appreciated. Cheers.

 

[egellings]

It can be tricky because input capacitance, called Miller capacitance, is depended upon the gain of the input stage.

 

[its_chicken!]

I have a bit more detail on what is outlined there on the schematic from the parts list and a different diagram if that is of any value:

From the specs:

From the printed circuit board and parts list with the relevant part numbers for this board below:

I'll attach the manual if anyone is interested in case I have confused things more:

https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Sansui-AU-505-Service-Manual.pdf

Cheers

 

[its_chicken!]

I will add that I'm trying to work out if a cartridge that requires a load capacitance of 100pf to 200pf would get along with the au-505 or if it will require a lower capacitance external preamp.

 

[Hyperion]

I would try it if you can and see how it sounds, as I don't think that information is available anywhere. If the capacitance is too high the most likely (but not always), indication is that the treble will be a bit subdued, sometimes the indication is much more subtle. Of course the signal cables from the turntable to the amplifier count in respect of capacitance - the longer they are the higher the capacitance. Also there are wide variations in the characteristic capacitance of some screened cable used for phono (and any), connections to the amplifier.

Perhaps the only way you are going to know if it sounds OK is, if you are familiar with how the cartridge/turntable should sound with an amplifier with known input capacitance figures available.

 

[its_chicken!]

I would try it if you can and see how it sounds, as I don't think that information is available anywhere. If the capacitance is too high the most likely (but not always), indication is that the treble will be a bit subdued, sometimes the indication is much more subtle. Of course the signal cables from the turntable to the amplifier count in respect of capacitance - the longer they are the higher the capacitance. Also there are wide variations in the characteristic capacitance of some screened cable used for phono (and any), connections to the amplifier.

I see, my concern was in fact the opposite, that the treble or high end would be too harsh or bright if the Sansui, in addition to cables from the TT, exceeded the capacitance range recommended for the cart I'm interested in which is the AT VM95ML. I'm only checking as I've read that the cartridge, along with other AT MM cartridges, are sensitive to capacitance loading.. but I think I'll give it a shot.

 

[Hyperion]

I think it depends quite a lot on the cartridge (and I am not an expert here), but normally the inherent phono cable/phono amp capacitance 'shunts' or 'shorts' higher frequencies to ground - the higher the capacitance the worse it gets.

 

[JoseHH]

I will give it a try, but I am open to criticism if my answer is off.
Capacitor 601 does not contribute because it is in series with the input, but then you have the miller capacitance of the first stage plus the series combination of C603 and C605 which are between the input of transistor Tr601 and ground.
Transistor 601 appears as a common emitter amplifier, so its gain at low frequency is G=Vout/Vin= - Rc/Re=180KOhms/(3.9+0.33)KOhms=-42.
Note that at high frequency, the 10uF capacitor on the emitter of Tr601 will start to become a low impedance and effectively it shunts the 3.9K resistor, which will increase the gain as the frequency increases. At DC the capacitor has infinite resistance, while at high frequency Zcap=1/Frequency*C. At 20KHz it will be a few ohms, so the gain becomes quite large, probably close to 400.
I could not get the datasheet for the transistor Tr601, and the only value I found for the output capacitance is 5 pF. So the maximum capacitance of the circuit, would be the sum of all the capacitances between the base of the transistor and ground, so we have a 68pF in series with the 10 uF capacitor, which would be effectively 68pF, and the miller capacitance, which in the worst case seems to be on the order of 200*5 pf~1070 pf. In the low frequency limit it will be closer to 300pF.
In the worst case, the 2.2K input resistance in combination with the 1070pF would form a low pass filter with a cutoff frequency of 67kHz.

 

[its_chicken!]

I will give it a try, but I am open to criticism if my answer is off.
Capacitor 601 does not contribute because it is in series with the input, but then you have the miller capacitance of the first stage plus the series combination of C603 and C605 which are between the input of transistor Tr601 and ground.
Transistor 601 appears as a common emitter amplifier, so its gain at low frequency is G=Vout/Vin= - Rc/Re=180HOhms/(3.9+0.33)KOhms=-42.
Note that at high frequency, the 10uF capacitor on the emitter of Tr601 will start to become a low impedance and effectively it shunts the 3.9K resistor, which will increase the gain as the frequency increases. At DC the capacitor has infinite resistance, while at high frequency Zcap=1/Frequency*C. At 20KHz it will be a few ohms, so the gain becomes quite large, probably close to 400.
I could not get the datasheet for the transistor Tr601, and the only value I found for the output capacitance is 5 pF. So the maximum capacitance of the circuit, would be the sum of all the capacitances between the base of the transistor and ground, so we have a 68pF in series with the 10 uF capacitor, which would be effectively 68pF, and the miller capacitance, which in the worst case seems to be on the order of 200*5 pf~1070 pf. In the low frequency limit it will be closer to 300pF.
In the worst case, the 2.2K input resistance in combination with the 1070pF would form a low pass filter with a cutoff frequency of 67kHz.

Wowie, that's impressive. As someone who's very new to this and only very slowly learning it's quite difficult to wrap my head around it all but I'm digging the process. That's helpful, thank you! So from what I understand from that is it is potentially quite a way off with the recommended 100-200pf loading for the AT cart?

 

[JoseHH]

As someone who's very new to this and only very slowly learning it's quite difficult to wrap my head around it

^^^^^^^ This applies to me, so I appreciate it if somebody can elaborate.

In any event, the input impedance is not exactly 2.2 KOhm, and yes you would expect some roll off at high frequencies. There is also another complication which I left out here, which is the RIAA equalization network, R621, R623, C613,C615, this feeds back from the second stage of this preamp, and it affects the frequency response, but I am not sure how it may affect the input capacitance. The gurus at the diyaudio forum would be the community to ask about phono preamps. Many of the members of that forum designed phono input circuits. Please check that forum up if you have the chance.