Croft Micro 25R channel imbalance

[Canetoad]

Recently the output from my main system has started to skew towards the left channel. At first I thought it was something in the recordings but, over time, it has worsened. Great! Something else has gone wrong!

Anyway, last night I checked first to see if it was the speakers by reversing them. Then the interconnects between the pre and power amps. No, not them either! I isolated it to the Micro 25R. I know the problem doesn't exist with the phono stage because I did a needle drop a few days ago and channel balance was fine. It's also fine through the headphone amp connected to the line-out of the preamp. Before pulling it out and opening it up to check the valves I decided to confirm again what I was hearing (obsessive compulsive side) by turning the volume down individually to listen to each channel in turn. When I turned down the right channel and turned it up again the problem disappeared. Bloody volume pots!

I was so glad it was something minor. :)

[Jimbo]

Recently the output from my main system has started to skew towards the left channel. At first I thought it was something in the recordings but, over time, it has worsened. Great! Something else has gone wrong!

Anyway, last night I checked first to see if it was the speakers by reversing them. Then the interconnects between the pre and power amps. No, not them either! I isolated it to the Micro 25R. I know the problem doesn't exist with the phono stage because I did a needle drop a few days ago and channel balance was fine. It's also fine through the headphone amp connected to the line-out of the preamp. Before pulling it out and opening it up to check the valves I decided to confirm again what I was hearing (obsessive compulsive side) by turning the volume down individually to listen to each channel in turn. When I turned down the right channel and turned it up again the problem disappeared. Bloody volume pots!

I was so glad it was something minor. :)

Interesting findings Bernie, I think I have the same issue with my 25R but not absolutely certain. It may be a slight discrepency with my valves as they are not matched pairs?

[zanash]

just read your post ...the obvious thing that struck me is that one of your valves is on its last legs ....should be easy enough to swap the chanels valves and see if the fault follows

[orbscure]

just read your post ...the obvious thing that struck me is that one of your valves is on its last legs ....should be easy enough to swap the chanels valves and see if the fault follows

+1 with Pete's suggestion. I had a similar issue with my Micro 25 Basic and did the same thing and found that one of my valves was faulty...

[montesquieu]

Croft gear is 'modestly priced' for a reason ... you won't find many boutique bits in these models.

[zanash]

don't disagree ...but the croft punches way above its weight so its certainly worth rolling a few tubes ....may even get a better sound

[DSJR]

Croft gear is 'modestly priced' for a reason ... you won't find many boutique bits in these models.

Bloomin' good job too :lol: Too many 'boutique bits' as fitted to some exotic US confections often go out of fashion as the years go on I recall. Careful selection of valves and maybe considering stepped precision attenuators is probably all you really need to do - I notice Glenn uses high wattage resistors in his products now as compared to the 1/2W Holco-style (Welwyn?) resistors in my old Series 4PP, but don't know if this was for production or consistency reasons.

[Sondekker]

Rather curiously I've been experiencing a similar issue with my Croft 25R only this time with the phono stage. In the last month or so, I've needed to turn the left volume control way further round than the right channel when listening to vinyl. Both volume knobs are turned to a similar degree when listening to CD through the line stage.

The valves in the phono stage are a matched pair of relatively brand new (six months old) valves and I've been too lazy to swap them over to see if it is indeed one of the valves that's already starting to misbehave although it does disturb me to see both controls set so differently when listening to LP's......

[DSJR]

I remember back in the day that valves only carried a ninety day warranty, even if the gear they were used in was guaranteed longer.

[OD1]

I had a similar problem when I put some balanced TJ FM 12AX valves (that worked well in my Icon Audio phono amp) into the phono section of my Micro 25. There was a large volume reduction in the left channel that moved to the right channel when I swapped the valves over, but they remained in balance when re-inserted into the Icon amp.
Never had that problem again after rolling 6 or so different pairs of valves, so maybe the fault in that particular valve only showed itself when in the Croft's circuit ?

[Firebottle]

If the phono circuitry is similar in the Micro to the 25R there may be a plausible reason for the effect.

The second stage in the Croft is run in 'starvation bias' mode, this is pretty unique from what I've seen in valve phono stages. This is done so that a high impedance RIAA passive network can be employed, amongst other reasons.

:cool: Alan

[walpurgis]

Tell me what "starvation bias mode" is Alan. You've lost me with that one.

[Firebottle]

Rather than rely on the current flow through a cathode resistor to develop the negative grid bias (with respect to the cathode), starvation bias uses a very high value grid resistor, usually 10M or more, so the very small grid current generates the bias.

Alan