Questions/Guenivere [message #30075] |
Sat, 01 January 2005 10:00 ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
Manualblock
Messages: 4973 Registered: May 2009
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Illuminati (13th Degree) |
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Can we figure the output impedance of the pre? If so, can someone volunteer to take the walking tour through the procedure of finding and figuring the numbers. Then how to adapt or figure the required input impedance for the eventual power amp. Thanks for bearing with these unreasonable demands with patience; J.R.
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Re: Questions/Guenivere [message #30076 is a reply to message #30075] |
Sat, 01 January 2005 14:04 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
Damir
Messages: 1005 Registered: May 2009
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Illuminati (2nd Degree) |
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If we use "normal", anode output, then output impedance of the stage is anode resistance in our working point (about 2,5 kOhms, or little less) in parallel with anode load (in this case CCS with about 1MOhms resistance). Then our Rout=ra~2k5. When we use "mu" (or Source) output, then our Rout is somewhat lower. I have no experience with depletion-mode mosfets and can`t say exact number, but I suspect not by much (simple CCS). The "rule of thumb" for the impedance "tuning" is at least 10:1, or we can "drive" about 25kOhms input impedance. That means every "normal" tube amp with Rin of 47k or more can be used with our preamp. And few words about actual building - IMO, it`s very easy to build nice oscillator, but not so easy "clean" preamp. I`d use resistor "stoppers" on everything. Grid stopper (say 1k) on the 5687 (solder close to the pin, no lead). Then, Gate stopper (1k), and our P1 substitute with required resistor (adjusted on test). This resistor (between the Source and anode pin) solder in a way that both leads are very short. I`d use resistor between B+ and Drain, say 220 Ohms, again - short lead to the mosfet. And finally, I`d use output resistor, between Source and C2, say 220 Ohms again (close to S). That resistor would raise output impedance a bit, but it`s a precaution against oscillations. Maybe I`m overly paranoid and have a bad luck before? Doug, feel free to jump in with your experience/opinion ... ![](http://www.audioroundtable.com/emoticons/smiley.gif)
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More... :-) [message #30077 is a reply to message #30076] |
Sun, 02 January 2005 03:58 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
Damir
Messages: 1005 Registered: May 2009
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Illuminati (2nd Degree) |
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![](http://img147.exs.cx/img147/1074/5687wa0bz.jpg)
Forgot to add... Source follower output has another good feature, it isolates our triode and the load (capacitive cable + low input resistance of the amp). If we have rel. high input amp (say, >100k), we can try anode output, too. Then our triode has load Rccs//Ramp - not so high anymore. Our DN2540 facing upward (in TO-220 case) has leads (from left to right) - G,D,S. Note that D is connected at the tab. We must use a little heatsink (see the picture, hosted by "ImageShack" :-)), and thermally conductive insulator. Better not to mount heatsink on the chasis - safety and negative influence of the stray capacitance. We`d use two series connected cheap LED, cathode (short lead) goes to ground.
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Output Z [message #30080 is a reply to message #30079] |
Sun, 02 January 2005 18:12 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
PakProtector
Messages: 935 Registered: May 2009
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Illuminati (2nd Degree) |
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Hey-Hey!!!, There are two things to account for the output Z. First is the series cpaacitor we are using for coupling. For a 5-mike we're looking at ~1k5 @ 20 cps. 1/[2*Pi*{frequency in cps}*{capacitance in farads}] if you feel like breaking out your slide-rule.I have seen measurements made by a trustworthy and skilled experimenter indicating that the mu output, at the currents we are looking at( 10 mA ) is granting output z in the few hundreds of Ohms. So the low freq performance is dictated by the cap, and at mid-band won't be worse than a few hundred Ohms. Makes silly-expensive patch cords less of a requirment, no? regards, Douglas
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Re: Output Z [message #30084 is a reply to message #30083] |
Mon, 03 January 2005 12:44 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) |
PakProtector
Messages: 935 Registered: May 2009
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Illuminati (2nd Degree) |
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The explanation, as I see it: The Kenwood L-07 pre-amp had optional 10m cables to connect to the L-07m monoblock amps. They claimed that the super-low output z of the pre made the 5000 pF of cable capacitance ( 39 and change feet worth...) a non-issue. It was capable of driving a capacitive load, or so the claims went. So the few hundred ohms will do just fine with a few feet of moderatly capacitive patch cords... regards, Douglas
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