Hi All,
I was thinking of changing the phono stage to Fet and increasing the gain for a Denon Dl-103. BJTs to Fets K170bl and J74bl. Worth doing? Just a crazy idea? Would need a little guidance of which fet replaces which BJT.
I was thinking of changing the phono stage to Fet and increasing the gain for a Denon Dl-103. BJTs to Fets K170bl and J74bl. Worth doing? Just a crazy idea? Would need a little guidance of which fet replaces which BJT.
34db. A couple fets are cheaper than good transformers. No room inside chassis for transformers, so a lot of connectors and solder joints to add outboard transformers. If it's too much of a hassle I could replace the phono section with a "Pacific preamp" (fet) circuit.
Can you get that much more gain changing two fets? You'll probably want at least another 20db gain for a DL-103.
jeff
jeff
I can get 10~15db more from changing the feedback loop in the line stage. I also plan to get more rail voltage by changing the main transformer tap to 100V and applying 120v to it. Yes that requires a bit of PS work but that's ok. I envision using a cascode bjt/fet output in the phono section and have 30V rails to work with. Salas shows a couple versions of the NJFet RIAA that gives 40~50 db output. The Hafler circuit is a bit different (push/pull) so not sure I could configure it to work that way. I was really interested in the PASS F7 amp circuit reconfigured for a phono stage with the RIAA in the feedback loop. Maybe I can get Salas or Mr Pass to respond to my questions.
Hi, it takes to fully simulate for a conclusive answer but I am afraid that those BJTs already provide more open loop gain than a JFETs modification would.
I have two phono inputs, so I may put a FET "head amp" on one input (for the MC) and use the other for MM. I just wanted FETs rather than BJTs in the phono stage (I'm really a tube guy). Just having some fun with the Hafler.
There's a nasty BP cap on the input...does it really need to stay (in the stock circuit)? IF so, I do have room for a Polypro between the jack and circuit board (what is the min. value I can getaway with...47K input?).
Thanks for taking time to answer a bunch of silly questions.
There's a nasty BP cap on the input...does it really need to stay (in the stock circuit)? IF so, I do have room for a Polypro between the jack and circuit board (what is the min. value I can getaway with...47K input?).
Thanks for taking time to answer a bunch of silly questions.
The input cap has to stay there because BJTs have base current that finds its way to the cartridge otherwise. Replacing it with a film PP cap will be an upgrade. 2.2uF should do.
Thanks again! Kind of thought so. It was one of the reasons I was looking into FETs, plus I like the sound of Fets over BJTs. I may still give it a go and will post my progress. I plan to update the PS, new regs, snubber, etc.
Low noise bipolars are more suitable for MC cartridge. You need to parallel few fets to acheive same noise level. I'm skeptical about the OP idea.
One of my main goals was to switch to a Fet input stage to get rid of the input cap. IMHO the worst place for a cap.
Well for an ac amplifier the input is exactly the right place for a DC-blocking capacitor to protect from DC offsets causing havoc - on the input you know the impedance the capacitor is protecting and can size it properly (not usually possible on an output). Of course a phono cartridge isn't going to put a DC offset on your preamp, but there is the other way round - if the preamp fails it might put 15Vdc on the cartridge winding, which could be a bad outcome, so people usually include them on a phono preamp input.
With low level phono signals you don't have to worry about capacitor distortion except perhaps for high-k ceramic (which are abyssmal), but even then you may get away with it with only microvolts of signal across the cap.
With low level phono signals you don't have to worry about capacitor distortion except perhaps for high-k ceramic (which are abyssmal), but even then you may get away with it with only microvolts of signal across the cap.
Yes Jack, Hafler did offer two pre-preamplifiers for the DH-110. The DH-111 and the DH-112, both of which fit inside of the DH-110. Either of these are installed in the Phono 1 input circuit so you would have both an MC input and an MM input (Phono 2). The instruction manual describes how to install each of these in the DH-110. I have a DH-110 but both of my phono inputs are still MM.
Ken
Ken
Thanks Ken, now I recall. I had it archived in an old copy of Multisim.
My youngest son (who now has 3 kids) has both the Hafler DH-110 and 220 that I bought at a HiFi show on Long Island in the early 1980's. So it was the DH-112 I eventually purchased, but never got an MC cartridge. Here's the schematic and a graph of the results. +23dB gain. With surface mount you could probably make it tiny!
My youngest son (who now has 3 kids) has both the Hafler DH-110 and 220 that I bought at a HiFi show on Long Island in the early 1980's. So it was the DH-112 I eventually purchased, but never got an MC cartridge. Here's the schematic and a graph of the results. +23dB gain. With surface mount you could probably make it tiny!
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Thank you very much. I bought my 110 kit in 1984 and used it for years. I sure never thought I would find any more info on the pre-preamps let alone the schematic. That doesn't look too hard to build.
Thanks again.
Ken
Thanks again.
Ken
Here;s a first cut on a 2x2 inch PCB. The SMT analog of the 2N4400, 2N4403 are the MMBT versions BUT these are obsolete even though Rochester Elec has tens of thousands. There are some needed adjustments to use the ZXTN2018F, ZXTP2027F which are about as low noise as you can go with an SMT bipolar:
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I suggest you ideally need another 30dB, to complement the 0.3mV of your DL-103.
That is not a very good performer, in its published form. 🙁