Powered Subwoofer With Pas2.......Driving Me Crazy!

Trying to hook up a powered sub with my Pas2 . Using a "y" connector out of main audio outs. Sometimes it works but usually I get a fuzzy- distorted sound out of the subwoofer. Not the sub because I hooked it up in my other "bedroom" system ( non tube) and works just fine. It's almost like by splitting the output on the Pas2 the sub isn't receiving enough of a line level signal. Anybody have subs with their Pas2's or3's? Anybody with thoughts or ideas please chime in.

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Daveinthedesert wrote:Trying to hook up a powered sub with my Pas2 . Using a "y" connector out of main audio outs. Sometimes it works but usually I get a fuzzy- distorted sound out of the subwoofer. Not the sub because I hooked it up in my other "bedroom" system ( non tube) and works just fine. It's almost like by splitting the output on the Pas2 the sub isn't receiving enough of a line level signal. Anybody have subs with their Pas2's or3's? Anybody with thoughts or ideas please chime in.

The PAS2 has a limited driving capability.
Is your sub transistorized ? And what s it's input impedance ?
When you connect to 2 amps via a 'Y' the load impedancs is further reduced as
the 2 amps are paralleled.
You might need a buffer between the PAS and your load(s).

Daveinthedesert wrote:Trying to hook up a powered sub with my Pas2 . Using a "y" connector out of main audio outs. Sometimes it works but usually I get a fuzzy- distorted sound out of the subwoofer. Not the sub because I hooked it up in my other "bedroom" system ( non tube) and works just fine. It's almost like by splitting the output on the Pas2 the sub isn't receiving enough of a line level signal. Anybody have subs with their Pas2's or3's? Anybody with thoughts or ideas please chime in.

a) You will need to do the impedance mod on the PAS, if you have not done so already.
b) Have tried driving the sub directly - no Y-connector? What happens then?
c) That pre-amp has two outputs. Why the Y-connector?

On the Impedance Mod:

To do the impedance mod - per the Dynaco literature:

1. Obtain two 100,000 ohm, 1/2-watt 10% tolerance resistors.
2. Unsolder and discard the two 510,000 ohm (green-brown-yellow)
resistors connected to the audio-output sockets on the back panel of
the preamplifier and replace them with the 100,000 ohm resistors.
Solder all connections.
3. Snip out and discard the two 62,000 ohm (blue-red-orange) resistors
on the PC-5 circuit board adjacent to eyelets 8 & 18. This completes
the modification.

Given the relative difference in cost, I would use 5% resistors and
screen them as well.

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I think the bottom line here is that the original PAS was really not designed for this style of operation, I guess kinda like trying to cram a V8 engine into a 4 cylinder engine bay
Even with the suggested mod, still not a perfect solution for the poor ole PAS!

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indeed! Nobody disputes the upgradeability of any PAS model, after all, that's why we sell those upgrade kits
But it is still important to know the PAS' limitations.

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One observation from 30,000 feet and many years of Dynaco:

The OP calls what he has a PAS2. Whereas it is possible that prior to his ownership someone did the impedance mod, and it is *JUST* infinitesimally possible that it had that mod from the 'factory', the real odds of an OEM PAS2 being modded are slim at best. But, all he has to do is pop-the-top and look.

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With alteration of the in-circuit load resistors, a stock PAS can be modified to drive impedances down to about 25KΩ. When making this modification, I found the best way was a frequency sweep, while viewing a low-power output of the actual power amplifier being driven, on an oscilloscope. All the load resistors would be removed and a 100KΩ potentiometer, connected as a rheostat across the PAS's output, would be varied to provide the flattest output in the bass. The resistance would then be measured and duplicated using precision resistors in parallel.

This procedure took into account the variations from manufacturer specifications, in the input impedance of the power amplifier, and variations in that impedance with frequency. If the user wanted to use a different power amplifier, the procedure would have to be repeated with that amp.

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The one and only solution that does not involve soldering gun and lots of work is to add an external buffer. A few of the buffers on ebay is ok ( are pure cathode followers), a lot of them are not ok.

One could wish for such a device , either as a circuit board for diy or mounted and ready or
mounted in a box with power supply. If done simple it could be quite cheap.

Any takers ?

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PeterCapo wrote:peterh, can you recommend a specific external buffer as you mention?

No. I build my own in an old FM-1 as i wanted a remote volume also.
http://n.manet.nu/fm1/index.html

But i do remember a thread here when one ebay device was found and used with
good results.

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PeterCapo wrote:Okay, hold the phone for a minute, folks...

The OP's main concern is the distortion he is getting from his subwoofer.  Could it be caused by DC offset at the PAS outputs?  A resistor might be able to be tweaked for a possible improvement in the impedance match.  But, if there is DC offset from the PAS and the powered subwoofer is having difficulty with it, then impedance matching is not the most immediate problem here.  Is it certain the input to the subwoofer is capacitor coupled?

A small 1:1 transformer would isolate any DC offset. A number of equipment maker contemporaneous with Dynaco used interstage transformers for this purpose - AKA converting an unbalanced input to a balanced output. They are not expensive. Actually IS transformers survived right into the solid-state era and solved some inconvenient problems for those just starting to compete with tubes in power and resilience - AR for instance.

Good catch!

Peter W. wrote:

PeterCapo wrote:Okay, hold the phone for a minute, folks...

The OP's main concern is the distortion he is getting from his subwoofer.  Could it be caused by DC offset at the PAS outputs?  A resistor might be able to be tweaked for a possible improvement in the impedance match.  But, if there is DC offset from the PAS and the powered subwoofer is having difficulty with it, then impedance matching is not the most immediate problem here.  Is it certain the input to the subwoofer is capacitor coupled?

A small 1:1 transformer would isolate any DC offset. A number of equipment maker contemporaneous with Dynaco used interstage transformers for this purpose - AKA converting an unbalanced input to a balanced output. They are not expensive. Actually IS transformers survived right into the solid-state era and solved some inconvenient problems for those just starting to compete with tubes in power and resilience - AR for instance.

Good catch!

good point !
Even better , a small 5:1 transformer could take care of some of the misalignment
Could even be mounted inside the PAS ( far from the mains transformer)

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It is clear that an external buffer (as Peterh has commented), or a new PCB with a cathode follower (model TCLA, Roy, for example), solve the problem of connection to computers or transistor equipment.
But this new output influences the loss of quality with its connection to the ST70 ?.
A greeting.

Pillo69 wrote:It is clear that an external buffer (as Peterh has commented), or a new PCB with a cathode follower (model TCLA, Roy, for example), solve the problem of connection to computers or transistor equipment.
But this new output influences the loss of quality with its connection to the ST70 ?.
A greeting.

No. A proper cathode buffer won't affect the sound in any negative way.

In fact i'd suggest connecting the buffer at the PAS output and then the
power amp and other stuff on the output of the buffer.

May I posit that a *good* match between speakers and amp(s) does not need a subwoof.?

deepee99 wrote:May I posit that a *good* match between speakers and amp(s) does not need a subwoof.?

That works only if the main speakers are full range. More-or-less a rarity these days.