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Old 23rd March 2009, 06:00 PM   #1
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Default How do i control volume on dynaco st-70

i need some help how do you control the volume on dynaco 70 here is what i have i have turn table then it goes thru bugal preamp then thru the dynaco amp then to the speakers were can i add the volume control and how do i do this here is a link to the amp and preamp please help me im a newbee to hifi

http://www.tubes4hifi.com/bob.htm

http://www.hagtech.com/bugle.html
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Old 23rd March 2009, 07:39 PM   #2
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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.
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Old 23rd March 2009, 07:55 PM   #3
rknize is offline rknize  United States
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Depends on the Dynaco. My VTA-70 has copious amounts of gain...too much in fact. 1.2V peak-to-peak at the input drives the amp just short of clipping. If you have already tried it and it is as loud or louder than you would ever want, then all you need is a pot.
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Old 23rd March 2009, 08:47 PM   #4
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burnedfinger what is a good preamp to use or build
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Old 23rd March 2009, 08:49 PM   #5
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Quote:
If you have already tried it and it is as loud or louder than you would ever want, then all you need is a pot.
True

Some of us like concert levels therefore a line stage or preamp is needed.
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Old 23rd March 2009, 08:53 PM   #6
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Quote:
burnedfinger what is a good preamp to use or build
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.
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Old 23rd March 2009, 09:16 PM   #7
dangus is offline dangus  Canada
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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.
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Old 24th March 2009, 12:29 AM   #8
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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.
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Old 24th March 2009, 11:47 AM   #9
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wow great help i really want to say thanks for all the input i am a newbe so i cant thank you enuff for all the ideas i do have one more question a freind of mine said that this pot would work check it out and let me know what you think

http://cgi.ebay.com/DACT-Type-23-Ste...3A1%7C294%3A50
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Old 24th March 2009, 01:19 PM   #10
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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.
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