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Old 6th July 2014, 06:26 PM   #1
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Angry Tube preamplifier with chip power amp

Hello everyone! It's been a while since i posted but right now i'm in a little predicament and i don't really know where the fault is (if there is any).
I built the preamplifier in the schematic below and it is followed by a LM3886 "typical application" circuit, the potentiometer end being connected to the preamp via a 3.3uF/ 250V capacitor.
I thought it sounds very hollow at certain bass notes, and it ruins some songs, which seems strange cause i never had this problem before with any amplifier except a full tube amp that had obviously too low damping factor for my IC-era speakers.
Now, since my last preamp that was far simpler but less worthy that drove anything from transistor amps to IC's i've been using headphones only and it's been a while. I have to also consider that i got used to the sound of headphones and that's why this thing seems strange.
I grabbed and dusted my scope and checked the amplitude after each stage doing a full frequency sweep every time. I adjusted all the controls to have precisely the same output at any frequency and same on both channels then gave it another listen. Great sounding, crystal clear, immensely detailed reproduction, but only as long as those 200-400Hz notes don't show up in any considerable amount. Those certain frequencies when they appear make everything sound way overbassy and like it's coming from an empty barrel. Now i know you're going to maybe blame the speakers but i've been using them for years now and this never happened...
I just don't know where to look. There are many possibilities and i don't know which is more likely to have the unwanted effect.
- the computer and sound card have changed since my last amp
- i never used cathode followers until now but now i was forced to by not having large value audio taper pots
- i *think* in my last LM38** i used a 47uF in it's FB circuit not 22uF as the datasheet says, maybe it could alter the way lows are produced
- could be that a cathode follower overloads (wouldn't that have shown on the scope?) which is said to have a nasty effect as opposed to a simple gain stage overloading ?
- something is not biased properly ? (but all is designed based on the tube datasheets and selecting the load line for most headroom)

PS: there's resistors imitating potentiometers in the diagram cause i do not have models for audio tapers in that software (where i also check everything before building it and checking it for real).
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Last edited by Bloodmist; 6th July 2014 at 06:29 PM.
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Old 6th July 2014, 06:31 PM   #2
rongon is offline rongon  United States
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In between V2 and V3, I see a tone control network, but I do not see any potentiometers in it. Did you have tone controls in this preamp and then defeated them by taking away the pots?

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Old 6th July 2014, 06:38 PM   #3
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no, as i said, i also use that program i drew with to simulate, and those resistors were placed to simulate audio tapers at half turn
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Old 6th July 2014, 06:40 PM   #4
Bigun is online now Bigun  Canada
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I'm just wondering if you have some low frequency stuff / feedback on the power rail ?
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Old 6th July 2014, 06:52 PM   #5
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Interesting point. You refer to the positive rail or the ground? The ground is a 1.5mm copper bar, each smoothing cap is wired to where that tube is wired as a "local star ground". I figured it would be ok but i'm open to any idea. I also went ballistic on the smoothng caps and power supply, as it is apparent in the schematic, but again anything's possible. So where do i look first?
The same ground bus (but made twice the thickness) serves the IC's which are not int the schematic. There, all that is high-current including the speaker returns go directly to the main supply caps, while the "pre ground" connections go at the last tube's ground. There's no PCB (yet) for the IC's. I think i should include them in the schematic.
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Old 6th July 2014, 06:58 PM   #6
Mooly is offline Mooly  United Kingdom
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Valves aren't my thing but just a thought... doesn't the passive network need driving from a low (as in lower than a cathode follower might give) impedance ?
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Old 6th July 2014, 07:16 PM   #7
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True true...
Well, a cathode follower with a ECC88 such as that *should* in theory have less than 100 ohm impedance at it's output, at least with those values. Had it been a different valve i would no doubt have reached the same conclusion as you did.
Then perhaps i should replace that follower with say a source follower with a sturdy mosfet like IRF840 ( i got plenty of those and used them as such)?
Question: if the impedance from the cathode follower was too high, my thinking says the mids and highs would start to roll off, which does not happen says the scope ?
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Old 6th July 2014, 07:39 PM   #8
Mooly is offline Mooly  United Kingdom
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O-Kaay, a couple more thoughts then. Isolate the network and fit an attenuator of the same nominal impedance as the network and see if the preamp sounds "neutral". You could also squarewave test. With the controls flat, there should (ideally) be no degradation of the signal. Perhaps try that at quite high level.
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Old 6th July 2014, 07:58 PM   #9
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You're right i should eliminate that tone control and replace it with a resistive divider to test which i'll start right away.
Hm.. meanwhile i'm starting to see a problem with the way the ground is routed in here, not exactly the best way. I think i'd better make it a full star ground just to be safe, or at least make separate ground paths for tube sections of V3 + 4 because... well, i have currents there opposed to each other which is really asking for trouble if putting them on the same ground wire with finite resistance. While it SEEMS to be linear in testing, it may behave badly with real music...
In lack of an actual photo, here's basically how it is:
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Old 6th July 2014, 08:06 PM   #10
Mooly is offline Mooly  United Kingdom
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I would doubt the grounding is the cause tbh. The signal currents are so small (unlike a power amp) that any modulation that the grounding layout causes would be very small.

Probably wouldn't be my first suspect

Try the attenuator and bypass first and see what that shows.
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