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How do I control volume on Dynaco ST-70?

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#2
If I read your post correctly you are wanting to take the output from your phono preamp directly into the dynaco stereo 70.

I doubt that you will have enough gain to get much audio if any out of the system. You will need a preamp in my opinion.

Your audio chain would be turntable feeding the phono preamp which feeds the preamp or line stage which feeds the Dynaco stereo 70 which feeds the speakers.

The preamp or line stage will have volume control in which to adjust for proper level.
 
#6
burnedfinger what is a good preamp to use or build
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This is a question that will draw a lot of fire. My personal favorite is the Aikido. The like the 6FQ7/5687 version. Some will say it has too much gain and to each his own. Its all in what you like/want. That is one of the reasons I recommended a line stage to use with the system.
 
#7
If you do have enough gain, a "passive preamp" may be all you need. At the simplest, this would just be a stereo volume control, but you'll probably want an input selector switch. This could be made for almost free using salvaged parts from a dead receiver or integrated amp mounted in a recycled case, or could cost vast amounts of money if you use fancy stepped attenuators and gold-plated hand-made solid brass knobs and wires insulated with the finest silk.

The Bugle output impedance is rated at 330 ohms, so almost any volume control pot should work, from 10k to 500k.

Searching for "passive preamp" or "passive preamplifier" should turn up some helpful information.
 
#8
Merickrc,

I find myself agreeing with dangus and think you should try a simple passive 1st. A passive can be executed at minimal expense and, if it does what's needed, the story is over.

Look at the ST70 schematic. Notice the 470 KOhm resistor right at the I/P. The 1:10 rule of thumb tells us that 50 KOhms is the largest safe value for the controls. Given the 330 Ω O/P impedance of the "Bugle" phono preamp, 10 KOhm pots. are safe. 😉

An individual control in each channel allows easy setting of both listening level and channel to channel balance. Mouser stock # 31VJ401-F costs $1.40 each and is quite sufficient for proof of concept testing. If the passive works out well, an upgrade of the controls can be entertained.

The ends of each pot. are wired between phono preamp "hot" and ground. The wipers drive the "hot" connections of the amp. Ground is common. Use SHORT, unshielded, braided, cables between the controls and the amp. Passives lack drive capability, which requires that cable capacitance be held down.
 
#10
That FleaBay item is 250 KOhms, which is too "tall" for service in an external passive box. Pass it by.

BTW, the Alpha Taiwan Carbon control I suggested is not junk. It's inexpensive and sounds OK. If the passive works out, you can consider upgrading to PEC (Canada) milspec hot Molded Carbon controls.
 
#13
merickrc said:
so wil this alps pot work

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=220343672023

thanks for all the help
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Yes, that conductive plastic control will "work". However, it's ganged stereo in construction and channel to channel balance is not alterable.

Given the SS "Bugle" phono preamp, I'd avoid "cold" sounding conductive plastic or metal film resistor controls. IMO, "warm" sounding Carbon is the way to go. If you want to buy something better than Alpha Taiwan, get a pair of DigiKey stock # KA1031S28-ND. Again, the inexpensive Alpha Taiwan controls are decent. If the passive concept fails to prove out, very little money has been lost trying low cost parts 1st.
 
#14
Eli Duttman said:




Look at the ST70 schematic. Notice the 470 KOhm resistor right at the I/P. The 1:10 rule of thumb tells us that 50 KOhms is the largest safe value for the controls. Given the 330 © O/P impedance of the "Bugle" phono preamp, 10 KOhm pots. are safe. 😉
Click to expand...

I use a passive pre but I'm using a 250K pot. I'd like to know more about your 10:1 rule of thumb and what sort of mischief I might be causing.

I'm could be barking up the wrong tree, but if the idea is that it's fine to add (or subtract) another 10% to a value, then you have lost me. The pot resistance, being in parallel, would favor very high values in order to stay within 10%, no?

Another question; when we talk about safe, we are talking about safe for the source right? Since the preamps and sources must work together between manufacturers, the source designers must build in confromance to some standard so as to be compatable, no? I selected my pot based on my observation that 250K was a common volume pot value for preamps and integrated amps. Is my assumption in error?

Thanks
 
#18
merickrc said:
so gingertube r you saying that the 10k pot idea wont work and if so what do you think i should do
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No, that's not what he is saying - 10K is less than 10% of the input impedance of the dyna amp and interactions will be minimal. A 10K pot will work fine, and in fact so will a 50K pot particularly if cables are kept relatively short. (HF roll-off due to cable capacitance can be an issue if the cable is too long or very high capacitance cable is being used.)
 
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