Sansui AU-719 vs Pioneer SX-1250

[restorer-john]

...I would love to see that, can you share what did you find?..

Basically, the 'character'* changed throughout the volume pot positional changes more so in the Pioneers of the era we are discussing. I put it down to non-ideal matching between the tone stage-the volume pots and the amp front ends.

Sansui in their AU-Dxx series, used high gain power amps with high impedance FET front ends and high resistance volume pots with essentially nothing in front (in direct mode). Once Pioneer moved to preamp-less front ends (in direct mode), their FR plots at various pot positions smoothed out.

CD gave the manufacturers an ideal low impedance, high voltage source and they no longer needed (or wanted) a noisy, mismatched gain stage in front of such a quiet source. Direct/bypass buttons appeared on everything as the preamp was no longer needed (except for the phono stage).

At the same time, traditional preamp out/power amp in jacks disappeared on the TOTL integrated amplifiers because the power amp was essentially a high gain amplifier, only needing 150mV to develop full power with a volume pot up front. And there was no high level preamp stage.

As I see it, all FR ratings and plots are specified at the full volume position (where the impedance mismatches don't occur). Trouble is, nobody listens at the full volume position. Most listening is done around 8o'clock to 11o'clock on the dial and that is due to CD (2v as opposed to 150mV of legacy line devices).

That's where FR deviations are more pronounced and I believe more easily heard, especially with the older gear at low(ish) volume positions.

*character meaning the deviation from perfect linearity (flat)- rollofs, boosts as evidenced by FR plots.

I no longer have plots I ran on gear that is long gone, but as time goes on, I collect new FR plots on gear I service/restore/rebuild/play with. If there's interest, I'll post them. There's never been a great desire for actual measurements on AK, so, in the interests of peace and goodwill, I tend to only post figures now and then, lest be known as an evil measurements junkie.

 

[Hipocrates]

Great stuff.... thanks!

 

[rcs16]

Since I have a 1250 on the bench I am going to look at the freq response of the pre-amp.
the 1250 has a buffer in front of the level/bal controls. The ones that do not have the buffer I would think could have freq response differences vs level, we shall see.

Anyone work on a Sansui AU-Alpha707 (AU-X901) before?

I find the Pioneer tuners a bit easier to align than the Sansui's, oops did I just say that in the Sansui forum....

Maybe it is the way they write the manuals. I do not see too much difference in the designs. Pioneer documentation was one of the better mfgs. I think Marantz is worse than Sansui.

I just did a 1250 tuner for the first time, they say to adjust for best o/p level of the detector as shown on the ACV. I was taught at Motorola to adjust a tuner (RF FE) for best quieting sensitivity. Adjust for a null in the noise.
Try it, use the speakers and your ears to quiet a tuner and also watch the ACV.
There are a number other tricks, such as tweeking the IF transformers for lowest mono thd.

 

[kevzep]

such as tweeking the IF transformers for lowest mono thd.

Yeah I actually do that, as well as peaking them out....it works very well...

Maybe it is the way they write the manuals. I do not see too much difference in the designs. Pioneer documentation was one of the better mfgs.

I believe you are correct, the Sansui procedures always involve Genescopes, so I basically use a combination of the Pioneer methods and procedures I have found work for other tuners...
As you say when it comes down to it, they're isn't a whole lot of difference in design.....Never done a Marantz, but I do have one coming!!

Try it, use the speakers and your ears to quiet a tuner and also watch the ACV.

for sure, I might try that.....good idea!!

 

[restorer-john]

Since I have a 1250 on the bench I am going to look at the freq response of the pre-amp.
the 1250 has a buffer in front of the level/bal controls. The ones that do not have the buffer I would think could have freq response differences vs level, we shall see.

A few years back, we discovered the significant HF roll-off in the Pioneer Spec1 preamplifier that varied considerably with volume pot position. You were in on that conversation too. The only position where the Spec1 had a flat response was at or near the 100% full volume position. Anything below that, it's response was poor. Several examples were tested by me and and another AKer.

There's a thread here someplace.

edit here it is: http://audiokarma.org/forums/index....ency-response-guidance-required.652054/page-5



Spec1 has buffer stages BTW.

 

[rcs16]

I remember that conversation. The spec one has a 4 gang volume control. I thought that it was the second gang that was causing this roll off and I was suggesting to bypass it, but we left it at that and it was never tried.

I think the genescope was a left over from the days of L/C filters in the IF and when folks did not have a THD analyzer, so they adjusted detector linearity by eye.

I have the new big filter ecaps in the 1250. I will run the freq response tests tonight hopefully.

 

[kevzep]

I think the genescope was a left over from the days of L/C filters in the IF and when folks did not have a THD analyzer, so they adjusted detector linearity by eye.

I've tried doing it by eye, then having a look with the distortion analyser, its never close enough doing it by eye...

 

[rcs16]

Just measured the left pre-amp on the sx-1250, tape play in to pre-amp o/p
it rolls off at 53Khz -1dB, 100KHz -3dB
with the volume set for a quarter or -42dB. what I consider a low level without waking up the house.
THD was 0.03%, pretty much about spec. Not bad for its age.

 

[restorer-john]

Just measured the left pre-amp on the sx-1250, tape play in to pre-amp o/p
it rolls off at 53Khz -1dB, 100KHz -3dB
with the volume set for a quarter or -42dB. what I consider a low level without waking up the house.
THD was 0.03%, pretty much about spec. Not bad for its age.

That's pretty decent.