Dynaco PAS driving class D amplification

Frode365

Frode365

Member
This is a multi leveled question which I hope can become a discussion to help me decide what to put together for our family room 2 channel setup.

The space that the setup will be going in is not real friendly to running wires, and my wife is even less friendly to running "visible" wires. The room will likely get a set of EPI M150's and/or ADS L810's. She would love the vintage look of say a PAS-2 pre amp but she would not like much more gear than that visible. Keep in mind this is just a setup for the family room/nook/kitchen area, this is not our main listening area as we have a setup in the living room as well as a dedicated listening space. So here is question #1.

Does anyone have any experience running a PAS (or other vintage tube preamps) to small Class-D amps? I have a set of Fosi V3 mono blocks currently, though am open to other options in that arena as well, I like that I could mount the amplifier discretely on my speaker stand and only have to run the RCA cable to the amplifier from the pre. There are power outlets right where the speakers will be, so concealing the power supplies should be fairly simple as well.

Question #2, is there a preference on PAS tube preamps? I know there are plenty of mods in the Dynaco world, but I would want to start with the best foundation, if its ay consolation I really dig the look of the gold face PAS-2, though I haven't shown the boss both the 2 and the 3 to see if she has a preference.

Thank you in advance for any and all feedback
 
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^^ exactly this,

there is a possibility of some DC output on the PAS2 and PAS3 that doesn't exist on the 3X. Easy enough to fix with an output coupling cap in the PAS. Not sure if the amp has one already but DC is usually undesirable on the output of a preamp. The 2 and 3 are the same other than cosmetics. 3X has the special tone control pots that will lift it out of circuit. Nice feature, but its a custom pot so if they go bad they aren't easy to replace.
 
I’ve tried running the PAS 3x to a pair of Fosi ZA3. A lot of background noise. When I bypassed the pre-amp, it was quiet. So I knew it was the pre-amp that was the problem.
 
I run a refreshed vintage Lazarus Cascade Basic preamp that has (4) 12AX7. The preamp output gets split to a pair Dynaco tube monoblocks on mid/treble, and an active low pass filter to a solid state amp on the woofers. Last fall I tried some of the cheap Fosi and Aiyima Class D amps with a 48V PSU on the woofers, but they were boomy, ill-defined, and lacked authority. Both my old B&K ST-140 and my newer Linn LK140 are much better on the woofers. This discussion makes me wonder if the preamp was simply a mismatch for the inputs of the Class D amps?
 
knotscott said:
I run a refreshed vintage Lazarus Cascade Basic preamp that has (4) 12AX7. The preamp output gets split to a pair Dynaco tube monoblocks on mid/treble, and an active low pass filter to a solid state amp on the woofers. Last fall I tried some of the cheap Fosi and Aiyima Class D amps with a 48V PSU on the woofers, but they were boomy, ill-defined, and lacked authority. Both my old B&K ST-140 and my newer Linn LK140 are much better on the woofers. This discussion makes me wonder if the preamp was simply a mismatch for the inputs of the Class D amps?
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am not familiar enough with the lazarus to know what the drive capabilities are of the output stage. If you have the factory specs you may want to see what the output impedance specs are.
 
M3apx said:
I’ve tried running the PAS 3x to a pair of Fosi ZA3. A lot of background noise. When I bypassed the pre-amp, it was quiet. So I knew it was the pre-amp that was the problem.
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could be noisy tubes or resistors, hard to guess.
 
Dynakitparts sells a PAS MOSFET Output Buffer Kit that will enable it to drive the class d amp correctly if your okay with soldering etc.
 
knotscott said:
I run a refreshed vintage Lazarus Cascade Basic preamp that has (4) 12AX7. The preamp output gets split to a pair Dynaco tube monoblocks on mid/treble, and an active low pass filter to a solid state amp on the woofers. Last fall I tried some of the cheap Fosi and Aiyima Class D amps with a 48V PSU on the woofers, but they were boomy, ill-defined, and lacked authority. Both my old B&K ST-140 and my newer Linn LK140 are much better on the woofers. This discussion makes me wonder if the preamp was simply a mismatch for the inputs of the Class D amps?
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Did the Fosi or Aiyima amps have Post Filter Feed Back designed into their circuits? The load may have been the problem if they did not have PFFB.
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Sliphorn said:
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Did the Fosi or Aiyima amps have Post Filter Feed Back designed into their circuits? The load may have been the problem if they did not have PFFB.
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I'm not 100% sure, but I don't think so. The Aiyima's were an a A07 Premium, A7 Max 2, and Fosi Audio TDA7498E. I kept the 48V supply and the A07 for shop use, and it's been fine for that.
 
mcgjohn said:
am not familiar enough with the lazarus to know what the drive capabilities are of the output stage. If you have the factory specs you may want to see what the output impedance specs are.
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I do not have any spec info to speak of, and they've been out of business for a while. Can't find a schematic either, but it drives my Dyna/VTA amps and Linn LK140 pretty nicely.
 
dannyr said:
Dynakitparts sells a PAS MOSFET Output Buffer Kit that will enable it to drive the class d amp correctly if your okay with soldering etc.
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Thank You, I will look into this, I can solder, not an expert on this stuff, but time to learn
 
Frode365 said:
I know there are plenty of mods in the Dynaco world, but I would want to start with the best foundation,
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I have over a dozen or so Dynaco amps and preamps, tube & solid state. A couple were "recapped" by previous owners who had no clue what they were doing because the units needed to be completely rebuilt. The "recap" jobs were band-aids at best.

I share this because if you want the "best foundation", whether you mod the PAS or leave it stock, it will need to be completely rebuilt from the ground up as if it were a new kit. New wiring, deep clean all pots & switches (not simply spray deoxit), new RCA connectors, etc. This is the only way to have a good foundation.
 
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