Hello to everybody !
I have a Preamp Dyna PAS 3x I never used yet, or just as inputs selector. I would like to buy a cheap, basic power amp to use it and to hear (finally !) the sound of this preamp.
I was interested in some amps like Rotel 850 or 870, but I think the Pass needs an amp with a high input inpedance (the Rotel 870 has a 2kohm impedance).
So perhaps somebody here can give me an advice for a good, basic, and cheap power amp to use with this preamp ? A power amp available in Europ, because I write you from France.
Thank you, and a great end of year for everybody 🙂 !!
I have a Preamp Dyna PAS 3x I never used yet, or just as inputs selector. I would like to buy a cheap, basic power amp to use it and to hear (finally !) the sound of this preamp.
I was interested in some amps like Rotel 850 or 870, but I think the Pass needs an amp with a high input inpedance (the Rotel 870 has a 2kohm impedance).
So perhaps somebody here can give me an advice for a good, basic, and cheap power amp to use with this preamp ? A power amp available in Europ, because I write you from France.
Thank you, and a great end of year for everybody 🙂 !!
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Hi JonSnell !
I found it here :
http://www.rotel.com/sites/default/files/product/manuals/RB-870BX-OM.pdf
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Hello arosalo,
you're right, but it is to be used with a tube preamp.
(if I post my message in the solid state forum, will I get a "As far as saw there is no transistor in a Pass 3x ?" 😉
Depending of the power needed, but a dynaco ST-35 could be a good starting point.
New kits are available at dynkitparts.com , used flows around on ebay .
If you need more power a ST-70 might be an option, available new from tubes4hifi.com et al. , used can be found on ebay.
And, don't use a transistor poweramp whith the PAS3, it's like using
bicycle tyres on a corevette!
Hi petertub !
The ST-35 or ST-70 will be the next steep, but for now I would like to hear this preamp easily, without having to find a not-so-common amp, or without help of my soldering iron ;-)
The ST-35 or ST-70 will be the next steep, but for now I would like to hear this preamp easily, without having to find a not-so-common amp, or without help of my soldering iron ;-)
Then look at abay ( and simular )
This st70 is from UK, reasonable freight, 220V ready. ( sorry it need extrenal transformer
or you need to replace the built'in one).Still it's reasonable cheap .
Don't bother with "rebuilt" or "modified" or "improved", they are most of the
time inferior and nearly always, not documented and no spares avail.
"Good working order" or a local repair shop is what to look for.
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If you want to keep it in the Dynaco family, the ST-80 vintage solid-state amp has a 100k input impedance.
Replace the power supply capacitor, and the two output capacitors that connect to the speakers will be, at least, the first thing I would do. Small caps on the PCB would be my second thing to do, along with checking resistor values.
For a laugh I once rebuilt a Dynaco 80 - the amp was like $45USD from Ebay - and put $50 worth of replacement parts into it. Not bad at all, and it had higher fidelity than I expected considering the year it was made.
Before:
After:
Replace the power supply capacitor, and the two output capacitors that connect to the speakers will be, at least, the first thing I would do. Small caps on the PCB would be my second thing to do, along with checking resistor values.
For a laugh I once rebuilt a Dynaco 80 - the amp was like $45USD from Ebay - and put $50 worth of replacement parts into it. Not bad at all, and it had higher fidelity than I expected considering the year it was made.
Before:
After:
Yes !!I like this idea, to keep it in the Dynaco Brand ! And in Europ, I think it will be more easy to find a cheap like this one. I didn't know this amp, and I like it ! 🙂If you want to keep it in the Dynaco family, the ST-80 vintage solid-state amp has a 100k input impedance.
Replace the power supply capacitor, and the two output capacitors that connect to the speakers will be, at least, the first thing I would do. Small caps on the PCB would be my second thing to do, along with checking resistor values.
For a laugh I once rebuilt a Dynaco 80 - the amp was like $45USD from Ebay - and put $50 worth of replacement parts into it. Not bad at all, and it had higher fidelity than I expected considering the year it was made.
You can't have your cake and eat it too! 🙁 A PAS needs to work into a high impedance, something like 1,000,000 ohms. Get your soldering iron out and build the requisite buffer.
A +15/-15 V. regulated bipolar PSU, a TL072 or more modern FET I/P dual opamp, and some passive parts get the job done.BTW, the same buffering issue is present, should you desire to record the O/P of the phono section.
TANSTAAFL!
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The PAS3 doesn't have much current drive due to the direct 12AX7 plate output. So RCA cables are limited to about 6' do eliminate high freq loss, and the amp input has to be higher than 100kohms.
dynaco ST80 and ST120 were designed especially for this high input impedance. The ST120 was somewhat unreliable due to cheap unfinned heat sinks on the output transistors, I own one and have rebuilt it with fans. Mine was $50 with burnt one channel. The ST80 has better heatsinks, but again like ST120 the crossover distortion exists when cold. I have requipped my ST120 with the Apex 6 board http://www.diyaudio.com/forums/solid-state/236256-retro-amp-50w-single-supply-20.html, which I have modified to have about 200k input impedance. This requires an input transistor of hfe greater than about 300, which is very cheap & common these days. My 2n5401 from On Semi had enough without special selection. See post 212 on p 22 of retro amp 50W for pictures of my modified ST120 with modified AX6 board. The modifications to the input resistors gets the input impedance up. Schematic is posted there, if you download it your viewer will blow the pdf up bigger. Use faster VAS & driver transistors than I did, one tinkly bell cut on an LP is suffering from slow slew rate. Otherwise the sound is superb.
If you want to stay with tube power amplifier, the ST70 kit from triodeelectronics.com of USA Illinois had US built transformers last time I checked with them. I own an original 1961 ST70 but find it difficult to purchase good tubes without supporting the invasion of Ukraine. That is why I am buying transistors, you can get them from Phillipines (ON) Mexico (ON) or Malaysia (fairchild, ON, others) who haven't invaded anybody lately.
The PAS3 sounds really good with the selenium rectifiers in the filament circuit replaced with 1n4001 to 4007, the electrolytic caps replaced, a good volume control, and possibly a rectifier tube. In my 1961 PAS2 I've replaced the volume control and rectifier tube twice, the electrolytic caps 3 times. The 12AX7 tubes are original and are fine.
I started repairing and modifying my Pas2 and ST70 in 1970 when I was twenty. The MacIntosh salesman at his clinic said my sT70 power was town to 7 W/ch due to probably bad e-caps, power tubes, and rectifier. He was right, and I had it up to 35 W/ch (17 VAC on 8 ohm speaker) within a year.
My modified ST120 sounds better than the ST70 even when the ST70 has a lot of new parts. 1% HD makes piano tracks sound a little fuzzy. Both the Apex6 side, and the original dynaco side with djoffe idle current control board sound better. I kept blowing sense transistors on the idle current accessory board is why I don't recommend it. The OEM PC15 driver transistors and VAS transistor make it sound excellent even on the tinkly bell track my TIP41C/42C driver and D44R2 VAS AX6 board sounds imperfectly on. This is at 1.5 Vac on my SP2-XT speakers at 8 ohm, using MJ15003 equivalent(NTE60) output transistors.
Have fun with the do it yourself hobby.
dynaco ST80 and ST120 were designed especially for this high input impedance. The ST120 was somewhat unreliable due to cheap unfinned heat sinks on the output transistors, I own one and have rebuilt it with fans. Mine was $50 with burnt one channel. The ST80 has better heatsinks, but again like ST120 the crossover distortion exists when cold. I have requipped my ST120 with the Apex 6 board http://www.diyaudio.com/forums/solid-state/236256-retro-amp-50w-single-supply-20.html, which I have modified to have about 200k input impedance. This requires an input transistor of hfe greater than about 300, which is very cheap & common these days. My 2n5401 from On Semi had enough without special selection. See post 212 on p 22 of retro amp 50W for pictures of my modified ST120 with modified AX6 board. The modifications to the input resistors gets the input impedance up. Schematic is posted there, if you download it your viewer will blow the pdf up bigger. Use faster VAS & driver transistors than I did, one tinkly bell cut on an LP is suffering from slow slew rate. Otherwise the sound is superb.
If you want to stay with tube power amplifier, the ST70 kit from triodeelectronics.com of USA Illinois had US built transformers last time I checked with them. I own an original 1961 ST70 but find it difficult to purchase good tubes without supporting the invasion of Ukraine. That is why I am buying transistors, you can get them from Phillipines (ON) Mexico (ON) or Malaysia (fairchild, ON, others) who haven't invaded anybody lately.
The PAS3 sounds really good with the selenium rectifiers in the filament circuit replaced with 1n4001 to 4007, the electrolytic caps replaced, a good volume control, and possibly a rectifier tube. In my 1961 PAS2 I've replaced the volume control and rectifier tube twice, the electrolytic caps 3 times. The 12AX7 tubes are original and are fine.
I started repairing and modifying my Pas2 and ST70 in 1970 when I was twenty. The MacIntosh salesman at his clinic said my sT70 power was town to 7 W/ch due to probably bad e-caps, power tubes, and rectifier. He was right, and I had it up to 35 W/ch (17 VAC on 8 ohm speaker) within a year.
My modified ST120 sounds better than the ST70 even when the ST70 has a lot of new parts. 1% HD makes piano tracks sound a little fuzzy. Both the Apex6 side, and the original dynaco side with djoffe idle current control board sound better. I kept blowing sense transistors on the idle current accessory board is why I don't recommend it. The OEM PC15 driver transistors and VAS transistor make it sound excellent even on the tinkly bell track my TIP41C/42C driver and D44R2 VAS AX6 board sounds imperfectly on. This is at 1.5 Vac on my SP2-XT speakers at 8 ohm, using MJ15003 equivalent(NTE60) output transistors.
Have fun with the do it yourself hobby.
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