| sv62 |
| Pls. recommend an ideal schematic for a tube rectified preamp (line stage only) that should work very good with rotel 1075 ss amp. They say a foreplay circuit should work fine but i dont have the circuitly. Would appreciate your help? Thanks alot! |
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| Eli Duttman |
The Foreplay is relatively high gain. SS power amps "typically" have a sensitivity of less than 2 V. for full power. That means the O/P of a CDP will drive the amp into clipping. So, you need a buffered volume control with approx. unity gain.
A "simple" cathode follower fills the requirement. A thread discussing this subject appeared on this "board" quite recently. |
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| fdegrove |
Hi,
| quote: | | A thread discussing this subject appeared on this "board" quite recently. |
This one you mean?
GAVIN'S 5687 FOLLOWER
A little bit of self-promotion never hurt anyone...so:
TUBE RECTUMFRIER A LA RIGUEUR...
The linestage is a White Cathode Follower preceded by a volume control...
That one can drive a SS amp input impedance and a mile of I/C cable...so to speak...Just what the doctor ordered.
Cheers,;) |
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| sv62 |
| Will the 5687 schematic be advisable on this pairing? |
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| fdegrove |
Hi,
| quote: | | Will the 5687 schematic be advisable on this pairing? |
Yes, quite likely it will do fine BUT....as it uses a dual rail supply AND you want to use a tube rectified supply, you'll now either a CT xfmr with FW rectification or use a FWB using vacuum diodes with a single ended xfmr.
Cheers, ;) |
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| G |
| quote: | Originally posted by sv62
Will the 5687 schematic be advisable on this pairing? |
If you have a SS amp with a high sensitivity input then it will be perfect. |
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| sv62 |
| Thanks guys! Based on ur recommndations, anybody out there wants to share the schematics? |
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| sv62 |
| How do you attain a maximum output impedance of 200 ohms to a tube rectified preamp? Would appreciate your help. Thanks again. |
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| Eli Duttman |
The O/P impedance of a cathode follower is approximately = to the reciprocal of the tube's transconductance (gm). The 5687 has a high gm; therefore, it has a low O/P impedance when wired as a cathode follower.
Voltage follower amplifiers: cathode (tube), source (FET), and emitter (BJT) have a high I/P impedance and a low O/P impedance. The voltage gain is slightly less than unity. The power (current) gain is considerable, as the "same" voltage that exists in the high impedance I/P circuit also exists in the low impedance O/P circuit. |
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| sv62 |
My ss amp has an input impedance of 33k ohms, what will be the ideal output impedance of the tube preamp? Some forums state that for this range of ss amp the ideal should be no more than 200 ohms for the preamp. Will it affect the sound quality if the preamp output goes below 50 ohms or 100 ohms?
I will be using 12au7 and 5v4 tubes & my chioces are national & hitachi, which do you recommend to be the best choice?
Thanks!!! |
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| Eli Duttman |
In the small signal situation, a driving unit should have a low O/P impedance and a driven unit should have a high impedance. The rule of thumb is that the driven unit's I/P impedance be at least 10X the O/P impedance of the driving unit.
Your power amp has a 33 KOhm I/P impedance. The unit driving it should have an O/P impedance < 3.3 KOhms. The O/P impedance of a 12AU7 cathode follower is under 1 KOhm; so, you should be fine.
Select a "low and hot" operating point. Relatively high plate current provides for better drive of interconnect cable capacitance and the I/P capacitance of the power amp. |
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| sv62 |
| Any recommendation on the tube brands? capacitors? & etc. Thanks. |
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| sv62 |
| just finished my tube rectified preamp attaining zero o/p impedance via cathode follower. however, i noticed that the sound vis my ss amp is still boomy. will an additional low-cut filter solve the problem? would appreciate your help. thanks. |
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