I need something to use as a preamp/volume control with my Dynaco ST70 series II. Budget is extremely tight, as is space. A tube preamp would be nice, and a kit would be fine, but I don't want to hunt down parts and try to fabricate a case or anything like that. I don't need phono or bluetooth or treble and bass controls. I also thought about just using a cheap DAC witha volume control on it instead of relying on the analog out from my CD/DVD player. I don't know if there is any such thing as a tube DAC or if such a thing would be affordable.
Anyone have any experience with any of these?
https://www.ebay.com/itm/115064545624?var=415147724758
https://www.aliexpress.us/item/3256804370199442.html
https://www.aliexpress.us/item/3256804364966465.html
https://www.aliexpress.us/item/3256804651307261.html
Other ideas?
Anyone have any experience with any of these?
https://www.ebay.com/itm/115064545624?var=415147724758
https://www.aliexpress.us/item/3256804370199442.html
https://www.aliexpress.us/item/3256804364966465.html
https://www.aliexpress.us/item/3256804651307261.html
Other ideas?
The first one on your list is just tube preamp, disipite it's description, it only accept audio input, while a true (tube) DAC accept coaxial, usb or bluetooth input. Non tube usb DAC preamp is cheaper (and maybe can opt for the DAC chip type you like).
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Very, very nice, but the $500 price tag is way too high. I don't necessarily need a DAC as I could use the D/A converter in my DVD/CD player, which isn't too bad for 2-channel audio actually. It has decent Burr-Brown chips in it. So I am torn between just a plain old tube preamp or some sort of DAC. I just need something between my DVD/CD player and my Dynaco ST70 series II because it has no volume control, and I just don't know what direction to take or what might be decent quality. The four links I posted earlier are all I have been able to turn up so far that might be suitable for the job and within budget contraints.
ST70 input is 1.3V rms to full output, that is something to consider, avoid preamp that output more than 3V rms.
And it also said input impedance should be less than 2K?
Just share this: I use variable output from my DCD player, which served me well for last 15 years until it retired recently. So I connect input to Line Out of sound card of my PC. Because my full range speaker needs more bass drive below 70Hz, it's very convenience for me to compensate with bass boost.
And it also said input impedance should be less than 2K?
Just share this: I use variable output from my DCD player, which served me well for last 15 years until it retired recently. So I connect input to Line Out of sound card of my PC. Because my full range speaker needs more bass drive below 70Hz, it's very convenience for me to compensate with bass boost.
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A DVD/CD player is likely to have max audio signal output of 2V rms.
Even though the factory spec for the input sensitivity of a Dyna ST70 is 1.3V to full power, it measured more like 1V to full power (https://www.audiosciencereview.com/...review-and-measurements-of-dynaco-st-70.7224/).
The input impedance of a Dyna ST70 is way up there at 330k ohms (https://www.stereophile.com/content/dynaco-stereo-70-ii-power-amplifier-measurements#:~:text=The input impedance of the,typical of many tube amplifiers.).
Since the output impedance of your DVD/CD player is probably about 1k ohms (or lower), I think this is a candidate for a 'passive preamp'. The problem to be solved is to keep the source impedance at about 1/10th the load impedance. 1/10th of 330k is 33k.
A potentiometer's maximum output impedance is 1/4 the value of the nominal resistance of the pot. So a 100k pot's output resistance is going to be a maximum of 25k ohms. You could use a 50k pot for the volume control for an output resistance of 12.5k ohms.
50k ohms will be an easy-breezy load for your DVD player.
The max output resistance of 12.5k ohms won't introduce any meaningful losses of the signal to the ST70 inputs.
If you don't want to build one of these things, there are a few that are commercially available, but they all use 10k pots for their volume controls. This will be fine also, as the DVD player's 1k output impedance will not be loaded down into a 10k ohm load. (It's still a fine load to source ratio of 10:1). It's also not a problem to have that low 10k resistance in parallel with the amplifier's 330k input impedance.
You may end up using the thing with the volume control up past half-way a lot of the time, due to the only 6dB of extra gain on tap. But that should still work fine.
A commercially available 'passive preamp' that doesn't cost a ridiculous amount of money:
Schiit Sys: https://www.schiit.com/products/sys
However, it's such an easy thing to make, I'd make one in a simple project box of your choosing, using a decent quality DPDT switch to switch between two inputs, six RCA jacks (two pairs for your two inputs, one pair for outputs), and the highest quality dual-ganged pot you can afford, or a stepped attenuator if you want. I built a cheap 'n cheerful passive vol ctrl box using one of these stepped attenuators from eBay: https://www.ebay.com/itm/384637419589?hash=item598e2d4045:g:I34AAOSwLqthyYjU. They sound fine and they have far better side to side matching than most potentiometers, even some pricey ones.
That should fall within your budget and get the job done. Just keep cable runs short between the vol ctrl box and your amp's inputs. Long cables will introduce unwanted capacitance that could cause problems. If you really need to have the preamp located far from the power amp, then you will need to start looking at a buffer, perhaps a cathode follower, to drive the added capacitance from long cable runs.
Even though the factory spec for the input sensitivity of a Dyna ST70 is 1.3V to full power, it measured more like 1V to full power (https://www.audiosciencereview.com/...review-and-measurements-of-dynaco-st-70.7224/).
The input impedance of a Dyna ST70 is way up there at 330k ohms (https://www.stereophile.com/content/dynaco-stereo-70-ii-power-amplifier-measurements#:~:text=The input impedance of the,typical of many tube amplifiers.).
Since the output impedance of your DVD/CD player is probably about 1k ohms (or lower), I think this is a candidate for a 'passive preamp'. The problem to be solved is to keep the source impedance at about 1/10th the load impedance. 1/10th of 330k is 33k.
A potentiometer's maximum output impedance is 1/4 the value of the nominal resistance of the pot. So a 100k pot's output resistance is going to be a maximum of 25k ohms. You could use a 50k pot for the volume control for an output resistance of 12.5k ohms.
50k ohms will be an easy-breezy load for your DVD player.
The max output resistance of 12.5k ohms won't introduce any meaningful losses of the signal to the ST70 inputs.
If you don't want to build one of these things, there are a few that are commercially available, but they all use 10k pots for their volume controls. This will be fine also, as the DVD player's 1k output impedance will not be loaded down into a 10k ohm load. (It's still a fine load to source ratio of 10:1). It's also not a problem to have that low 10k resistance in parallel with the amplifier's 330k input impedance.
You may end up using the thing with the volume control up past half-way a lot of the time, due to the only 6dB of extra gain on tap. But that should still work fine.
A commercially available 'passive preamp' that doesn't cost a ridiculous amount of money:
Schiit Sys: https://www.schiit.com/products/sys
However, it's such an easy thing to make, I'd make one in a simple project box of your choosing, using a decent quality DPDT switch to switch between two inputs, six RCA jacks (two pairs for your two inputs, one pair for outputs), and the highest quality dual-ganged pot you can afford, or a stepped attenuator if you want. I built a cheap 'n cheerful passive vol ctrl box using one of these stepped attenuators from eBay: https://www.ebay.com/itm/384637419589?hash=item598e2d4045:g:I34AAOSwLqthyYjU. They sound fine and they have far better side to side matching than most potentiometers, even some pricey ones.
That should fall within your budget and get the job done. Just keep cable runs short between the vol ctrl box and your amp's inputs. Long cables will introduce unwanted capacitance that could cause problems. If you really need to have the preamp located far from the power amp, then you will need to start looking at a buffer, perhaps a cathode follower, to drive the added capacitance from long cable runs.
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Excellent information. I was always told that a "passive preamp" was a bad thing and that there needed to be some sort of buffer. Adcom made a passive one that I had about 30 years ago.
Interesting, and affordable.
Even simpler, I don't need but one input. I have about 150 CDs and that's all I want to listen to. In other words, a volume control, 4 RCA jacks, and a little chassis. I wonder where I can get a quality, tiny little chassis. I'll look online. Minimal drilling required - one hole in the front and a few on the back. I definitely would want to put some lead weights inside to keep it from sliding all over the place.
About that stepped attenuator - I see lots of them with full-size resistors packed all around the switch. Are those any better? Some get quite expensive. What about one of the ALPS pots? Sometimes having notched volume controls is annoying.
With the ST70 ii inputs on the front, I always end up with about a 3 foot run to get from the shelf next to it or above it to the front of the amplifier. The front inputs always have annoyed me, but there is no room on the back, and the signal flows nicely from front to back the way the amp is laid out. I actually gutted it about a year after I bought it (new) in 1993 and replaced all of the components. The way it came from the factory, every time a tube went bad, it burned up at least one resistor on the board and often others nearby. What a pain in the neck. So, I used a little piece of fiberglass that I had on hand and wired everything point-to-point underneath it. Haven't had a problem since. I put better power supply caps in it and had a machine shop make a new metal cover plate for it because the caps were larger diameter. I think they charged me about $10.
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I had totally forgotten about this thread. Wow that was a long time ago as far as the overhaul of my audio system.
Update: https://www.diyaudio.com/community/...pot-on-chinese-dac.396291/page-4#post-7439220
I ended up with two of those because I liked the first one so much, with modifications mentioned in that thread.
https://www.aliexpress.us/item/3256802716154447.html
I also have one of these on order for testing:
https://www.aliexpress.us/item/3256803488001195.html
I then went on to build two new tube amplifiers:
https://www.diyaudio.com/community/...s-before-i-build-maybe-during-if-i-do.394962/
and
https://www.diyaudio.com/community/threads/new-dynakitparts-st-35-build.399961/
Update: https://www.diyaudio.com/community/...pot-on-chinese-dac.396291/page-4#post-7439220
I ended up with two of those because I liked the first one so much, with modifications mentioned in that thread.
https://www.aliexpress.us/item/3256802716154447.html
I also have one of these on order for testing:
https://www.aliexpress.us/item/3256803488001195.html
I then went on to build two new tube amplifiers:
https://www.diyaudio.com/community/...s-before-i-build-maybe-during-if-i-do.394962/
and
https://www.diyaudio.com/community/threads/new-dynakitparts-st-35-build.399961/