Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shooter on May 27, 2015, 01:58:47 pm
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I'm putting together a mono HIFI from leftover pieces, nothing anal audiophile, just something tube for music listening.
I’m finalizing the schematic and would like some help on the input section.
The 1st thing i’m sorta hung up on is blending/combining the L & R signals into mono?
Can someone point me at “how to examples”, I’ve found a few. I want it basic – simple to start, passive. Also, is there much difference mixing before or after a gain stage?
After the mix, I want a band pass filter?. I’m using this page;
http://www.sengpielaudio.com/calculator-timeconstant.htm (http://www.sengpielaudio.com/calculator-timeconstant.htm)
started with 22K as my Fcutoff, the calculators say that’s ~ 7.2uS
Then I plugged in R and C values to give me 7.2uS and came up with 720 ohms, .01uF
But, I’m hung up on how R & C are constructed, parallel with each other in series with the signal?, R in series, C to ground?, C in series, R to ground?
Anyway, thanks for any help
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Won't the NFB loop negate the tone controls and any bandpass filter you put in there?
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The things you guys find online never fails to amaze me.
On that calculator site, try "RC filter cutoff frequency . . ." I think this will make a lot more sense.
Once you have hooked a low-pass filter to a high-pass filter, you can find the resonant frequency by using the calculator "Geometric mean-Arithmetic . . . "
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thanks 2deaf,
Won't the NFB loop negate the tone controls
I was gonna have the mix and filter before V1, but on the TS I don't know enough to know. I think of NFB as a gain limiter, so I don't know how it effects frequency.
I used the freq cutoff calc to get the 7.2uS for 22K, then scrolled down, started plugging in R & C til I matched the 7.2uS TC. So I'm comfortable with the cutoff point, where I'm floundering is; do I call it a lo-pass, hi-pass, or band-pass. my guess is it'll be a lo-pass configuration, everything below 22K come on thru?
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> blending/combining the L & R signals into mono? ... ... ... mixing before or after a gain stage?
What IS your stereo source? If it is weak, boost first. If it is CD or iPod or any other device with a strong output, just tie two resistors together.
> After the mix, I want a band pass filter?
Why? By the time you get it to play it, most non-audio frequencies are long gone. Everything over 20KHz is chopped-off by digital reconstruction (or phono-needle cutoff). Bass below 50hz is often cut in the mastering.
> I think of NFB as a gain limiter, so I don't know how it effects frequency.
It gives the "same" gain at all frequencies. So you turn that Treb knob up, the NFB counteracts the change, treble hardly changes.
And for "hi-fi" the NFB you really want is from the loudspeaker back to an appropriate point before the final stage. This isn't guitar where we want lively self-resonance in the speaker. You want a very low output impedance. Like 1/10th of speaker impedance. The naked-pentode plan you show, the output impedance will be about 10 times the speaker impedance.
I do not see any reason to have a White Cathode Follower between a gain-stage with 150K plate load and the grids of EL84 with 235K grid resitance (and why two resistors?).
Ah- the WCF is "needed" to drive the NFB loop which foils your tone controls.
Steal the Fender Champ AA schematic. Move the volume control forward to the first grid. Change the tone-stack to a James network stolen from any old mono hi-fi. On test, reduce the NFB resistor from speaker to driver cathode to get appropriate overall gain and smooth polite tone. Sure you can use your two-EL84 in place of a 6V6.
125GSE is a *monster* lump of iron, costly to buy, costly to ship. IMHO the lesser 125ESE is not strained working at higher powers than two EL84 can muster.
OTOH one 12W output tube with a "Fender Champ replacement" OT, in the Champ AA circuit, will not be a less "fi" sound unless you really need that last 5 Watts output.
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Thanks PRR!
the original schematic with nfb and white CF was sans TS, I just inserted a bogen TS that looked interesting. The iron, tubes, chassis, etc are all *leftovers* that will go on Doug's 12" stout? chassis.
I was toying with the idea of KT88 but the iron I have eats up most of the chassis!
The drive will be from recorded, good level sources. My main reasoning for a "pre-filter" was to eliminate any HF noises on the audio before it gets to V1 and just a refresher on knowledge I forgot looong ago.