Vintage Amp...too much gain on CD

[waam68]

Hi
I have a Heathkit/Daystrom S99 integrated (9wpch push pull). It's my first valve/tube amp project & was dead when I got it. So far I've tested/checked all tubes/new mains cable/replaced almost all of the caps including the mains smoothing can and fitted a few ebay russian PIO & Teflon caps. The amp is sounding really good driving my old 15 ohm wharfedales via the matching tube tuner & with my turntable but there is noticible distortion on CD which I can feed in via aux or tape inputs (it is listenable via Radio or Aux inputs but distorted). I know this amp wasn't designed for the level of voltage a CD player kicks out. I have the manual & circuit diagram...& assume it's possible to change the resistor values on an input to alter the gain? or is there more to it?
Any guidance appreciated.
Mike

[HollowState]

Hi Mike,
Rather then making any changes that alter the circuit operating points to affect gain, I would make a simple two resistor voltage divider at the input to reduce the level of the incoming signal. This coud be done right at the input connector. It's like installing a fixed level control right at that point. You could even add a small toggle switch to have it in the circuit or not.

I assume this amp has a phono stage since you mentioned using a turntable. Do not replace any capacitors in the RIAA equalization stage with modern plastic or teflon types. To do so will alter the response curve even if you use the same exact value part. This is because plastic/teflon dielectrics respond differently to higher frequencies then the old paper or ceramic types. The high end will be accentuated. Even if you like the sound, it may be technically and musically incorrect. And the curve may have drifted anyway do to age, so don't make it worse.

Even changing capacitors further upstream could affect the overall system RIAA curve since it was probably tuned to work with the rest of the amplifier. This is always a touchy area when recaping a tuned circuit.

Victor

[gingertube]

Mike,
I would "choose" to use the AUX input for CD and then modify its input sensitivity to suit. As you suggested, that probably only requires the change of a resistor value or possibly the addition of a series input resistor, most likely right at the input.
Can you scan the relevant part of the schematic and post it? We could then offer more informed advise.
Cheers,
Ian

[Eli Duttman]

waam68,

This is a fairly common problem. Fortunately, the solution is reasonably simple. Commercial CDPs are capable of driving the IHF "standard" 10 KOhm load, which is fact you will take advantage of.

The concept of attenuating the signal from the CDP is correct. However, the place to do this is not inside the tubed unit. You build resistive voltage dividers into the RCA plugs that connect the CDP to the AUX I/Ps. 1/8 W. 1% tolerance metal film resistors are tiny and easily fit into the plugs. The total resistance used should be close to, but not less than, 10 KOhms. A combination of 6.98 KOhm and 3.09 KOhm parts will give you 10.26 dB. of attenuation. Connect the 6.98 KOhm part between the center pin of the plug and the cable's "hot" wire. Connect the 3.09 KOhm part between the center pin of the plug and the plug's outer "ring". The cable's "cold" wire connection to the outer "ring" of the plug is not altered.

Change the values of the resistors in the divider to modify the attenuation yield. For instance, 2X 5.1 KOhm parts in the divider yield 6 dB. of attenuation.

[waam68]

Thanks all for the tips
I am attaching a copy of the circuit. I'm a little confused with mechanically how I attach the resistors to the RCA plug...solder to the plug? wouldn't it be neater to just change a resistor value on an input internally?

With regard to the don't swap the paper caps for film etc it's already too late as I have recapped virtually all except for the silver Mica's & some of the ceramics, mostly with elec or Polypropylenes. Cap C21 .05uf are now Russian Teflon & C23 & C25 .1 uf's are Russian PIO. I also changed the preamp board ceramics for polystyrene. I wasn't worried about the originality aspect as someone had already changed quite a few to "orange drops" & others. I wanted reliability so did a "year zero" on the caps including the mains smoothing can & getting rid of a lot of old Hunts mouldseals. The sound is now noticibly smoother, bass & top more controlled. Possibly lost a little mid range warmth but it isn't a hard sound at all & from what I have read the PIO's will burn in a bit. Re the RIAA curve this amp has a variable pot on the Gram inputs. The only downer is I still have some hum with the volume up beyond 2 o clock (above my normal listening level), not bad but annoying! Any ideas?
Thanks
Mike

[waam68]

mmm don't seem to be able to attach the pdf!

[hnb2907]

Originally posted by waam68
Hi
I have a Heathkit/Daystrom S99 integrated (9wpch push pull). It's my first valve/tube amp project & was dead when I got it.

Hi Mike,

That was exactly my first project, and I still have my S99, currently driving a pair of 8ohm Rogers LS2's.

I found exactly the same as yourself regarding the CD input level. What I've done is to fit a potential divider, as suggested, on the back of each L&R phono sockets to drop the input level down to something reasonable. Off the top of my head I've no idea what values I used, but I can tell you that it's done on whichever input the right-hand selector switch is for (AUX?) My amp is a few miles away at my workshop, so let me know if you want me to find out what resistors I used.

I also replaced all the caps, the amp did work of a sort beforehand, was very quiet and muffled, which was immediately fixed. The only cap I haven't found a replacement for is the twin electrolytic on the B+ PSU - did you find a source for these? The only ones I can find seem to be either far-out wrong values or way too tall for the chassis... I also think this one is unwell or needs reforming?

I'd be interested to see the schematics and the instructions on biasing the output stage, if possible...


Cheers,
ChrisC.

[waam68]

Hi Chris
I used a JJ twin 100uf smoothing can which cost a tenner. Seems to work fine although I still have some hum with volume > 2 O'clock. The can fitting was slightly different so I took the original can off it's mount & superglued the JJ to the original fitting. Looks really neat. I'd be interested to see how you mounted the resistors & what value. I have the schematic & manual which I could copy. I've tried posting the pdf of teh schematic on here but it won't seem to take it ...possibly because I'm a "new" member?
Mike

[hnb2907]

Hi Mike,

Please send me an email on cartwric (at) hnb2907 (dot) co (dot) uk I can't seem to PM you through the forum?

I'll get you some details/pics of how I modified the input on mine - it wasn't scientific, just picking R values until the signal sounded and looked ok on a 'scope. I might not be able to reply until late tonight or tomorrow night...

BTW; what happens if you turn the bass dial past about 3:30pm, and play some music that has sustained bass notes (i.e. organ music, bad choice I know!) - Mine seems to be fine until it hits a certain volume, when the bass drivers in my speakers start to go wild. That's why I was wondering about biasing and replacing the PSU cap...

Cheers,
Chris.

[Original Burnedfingers]

Since some CD players have more level and some CD's play louder than others I would suggest putting a 10K pot on the input to properly adjust the level to suit the CD you are playing.