How to adjust my circuit for frequency response?

[n1np]

As I stated in the photo gallery thread, this is my first strictly audio project and my first time building with tubes. The amplifier is up and running and I have been listening for a few days now to a wide variety of sounds to get an idea of what changes need to be made.

The first issue is a bit of 60Hz hum, not noticeable a few feet away from the speakers but it is there. I have a couple of 470uF snap caps I can add on the output of the HV power supply filter: I expect that will help a lot. Speaking of filtering, I have seen many schematics (mostly radios) that use capacitive or LC bypassing on the heater connections, presumably to keep HF out of the heater supply. Is this done, or even useful, for audio frequencies?

The second issue is that higher frequencies seem weak. This is a subjective analysis based upon listening to a variety of music and comparing to listening to the same signal source directly with headphones. I started off listening to what I would listen to most often: dance, pop, and techno. My first impression was that everything sounded too bassy, and gain at about 1/3 made the floor vibrate. Heavy bass actually sounds good for those types of music, but it seemed that the high end was being covered up or maybe just weak. Next I listened to some solo piano recordings. They all sounded like they were recorded at a lower level and even with the gain turned all the way up the output was just not loud enough. Then I listened to some orchestral pieces. The drums and lower brass were overwhelmingly loud compared to the violins and flutes.

So my question is: what is happening to those higher frequencies? Looking back to my schematics, it occurs to me that C8 and R13 on the cathode of the buffer/splitter stage may be acting as an attenuator, at least for half the signal cycle. Am I thinking about that correctly? What else in the circuits affects frequency response like that? I would like to understand some before going in and swapping components around.

For reference, here are links to the schematics:
Schematic: Power Supply
Schematic: Preamplifier
Schematic: Buffer/Splitter
Schematic: Finals

Thank you for your help,

Ben N1NP

[analog_sa]

R10,R9,R8,C3 form some type of correction network which certainly attenuates the highs. Do you know what was the intended source? A very retro circuit. Why not build something modern?

[Shoog]

Look far more complicated than necessary.

R10,R9,R8,C3 form some type of correction network which certainly attenuates the highs. Do you know what was the intended source? A very retro circuit. Why not build something modern?

I would try taking these components out f circuit - one by one - to see what effect they are having.

Shoog

[analog_sa]

Strange circuit indeed. Ra on the input tube (12AX7) is a 5k pot!

Shorting R8 and removing the rest will certainly work but it's still a very silly way to design a pre.

[n1np]

The intended source was not specified, the circuit was described as a 'hi-fi preamp'.

I did deviate from the original design by changing R4 from 72k to 100k to bring the gain down to what seemed like a reasonable level. I also changed R2 and R11 from 3M to 680k because I had read somewhere that using such high values could cause low-frequency instability.

Are the R8, R9, R10, C3 then forming a low-pass filter? I admit that I could not find a design like that anywhere else to compare to.

Thanks for the replies,

Ben N1NP

[n1np]

After looking over the circuit some more, I decided to take out C16, the 0.001uF bypass cap on the cathode of the preamp output. The higher frequencies are now noticeably stronger and more detailed.

I will consider removing the R8, R9, R10, C3 network if I decide, after some more listening, that removing C16 didn't buy me what I wanted. I admit to not understanding what that network does and am hesitant to throw it out just for that reason.

Can anyone else shed some light on this? Thanks again for your help,

Ben N1NP

[Tom Bavis]

Subjectively stronger bass may result from a low-frequency peak in the speaker impedance - without negative feedback, the output impedance will be quite high, and it will deliver more power at frequencies with higher load impedance. You'll need more gain, but there is potentially PLENTY with a higher load for the 12AX7...

The biasing of the phase splitter seems strange, but I'd need to print and combine the pages to see waht's going on.

[SY]

Chances are that this was for a phono stage or a tape amp and the network was for equalization. Just substitute a normal RC coupling network and you'll be good to go.

[ray_moth]

That's the first time I've seen a pentode used as a concertina splitter. I wonder why anyone would do that? It seems a counter-productive move to make from the usual triode. I notice the two resistors making up the split load are unequal, I suppose to try to allow for the screen current being present in the lower resistor but not in the upper resistor, but I doubt if that would give enough accuracy of balance.

[n1np]

@ Tom Bavis: I realise that I have to consider the entire system of media, signal source, amplifier, room, listener, in my subjective analysis of what I hear.

FWIW, the signal source is a Fisher PCD-6900 cd player, far from hi-fi quality; the speakers are Yamaha NS-M125, which I got cheap and looked up today (from the spec sheet):
"NS-M125 speakers are designed for use as front or surround speakers in a surround sound system
Impedance: 6 ohms
Frequency response: 70Hz to 35kHz (no graph available)
Nominal input power: 40W
Maximum input power: 120W
Sensitivity: 86dB/2.83V/m
Crossover frequency: 6kHz"

It sounds like these speakers may not be a good match for the application. Having said that, it is sounding pretty good to my ears right now, but I need to get some other ears to listen as well. And you mention needing more gain, do you mean to drive the higher frequencies or to get a greater output sound level (it is loud enough already)? And I saw a design in a GE book that used global negative feedback, but do I really need that extra complexity?

@ ray_moth: Why would anyone do that? I designed it that way because I didn't know what I was doing and didn't know better . Could you explain to me better why that design is bad? I really am doing this project as a learning experience and could use some voices of experience explaining why I messed up!

I am way beyond the point where it occurred to me that I should have just copied a well-known working circuit, but at that point it was too late to stop and I have no regrets.

Thanks,

Ben N1NP