A Few Small Post-Build Issues

[williaty]

Issue 1:
A couple of the voltages are just barely out of spec. I've created a Google Docs Spreadsheet of the resistance and voltage checkout results to make it easier for you all to look at them. I've got three voltages that are just a little wonky. I'd be inclined to leave them alone, but I'm doing the Speedball in a day or two and want to make sure everything is right before I do it.

Issue 2:
The 6080 tube is HUGELY microphonic. I was hearing footsteps from around the house. I started tapping around the amp with my fingernail and an extremely ginger tap on the 6080 resulted in church bells ringing inside my head and my ears bleeding. Is this normal for the 6080 or is this an unusually microphonic example?

Issue 3:
I have WAAAAAY too much gain. With the 0-position of the pot being set straight down to 6-o'clock, I can't listen to anything above 9-o'clock. In fact, I'm usually listening at about 7 to 7:30. This puts me down on the portion of the pot that doesn't track well. I remember inputs/outputs can be padded to reduce signal level but I have no recollection of how to make smart choices of how to do this. Any suggestions?

Thanks guys!

[Doc B.]

You did see in that manual that +- 15%  on the voltages is fine?

The 6080 can be microphonic. That combined with your description of a hair trigger volume control has me wondering what headphones you are using. It would seem they are particularly sensitive.

[Grainger49]

What source are you feeding the Crack with?  It might have more output than is needed in combination with Doc's suggestion of sensitive headsets above.

Bottlehead has to make the kit so that it will function with a wide range of inputs and headphone sensitivities.  There are things to do if you determine what is causing the problem.

[williaty]

Oh! I thought tolerance on voltages was +-10%. I'm cool then!

I'm using Sennheiser HD-600s and the source is a High Resolution Technologies MusicStreamer II+ (2010 model). The MSII+ swings 2.25V FS. Given that I'm feeding it directly from the source files with no volume control on the computer, it's probably getting up close to that on a fairly regular basis, especially with modern recordings that are over-compressed.

[Grainger49]

That is a quarter of a volt higher than the CD "standard."  You could put a passive volume control before the Crack if you don't want to modify the crack. 

Most of the Bottlehead volume controls are 100k ohms, not the FP III.  If you put a 220k ohm resistor in series with the wire from the input RCA jack and the volume control it will pad it down considerably.

[williaty]

Yeah, I figured I was going to need to pad either the input or the output. Is a single resistor the way to go, or should I make an L or T pad?

[Grainger49]

I think the L-pad adds only one resistor per channel.  That is simplest, I like simple.

[Viktor]

Are you sayint that the output voltage from the MS2 is too high for the amp? Because I've been running it for about a month without any mods....Should I lower the input voltage or something like that?

[Grainger49]

Viktor,

It depends on your system.  If you use the same source and get good volume control then there is no worry.  What Williaty has is a problem that he can only use a very small turn on his volume control.  To make small adjustments at low listening levels, or any listening level, the control should, ideally, be at full clockwise rotation with your lowest input and your highest listening level.  No one gets that ideal situation. 

What most of us have is a good listening level at 12:00 to 3:00.  That works well and for late night speaker listening, or say background listening with a headset, you can make very fine adjustments of the volume control and get what you want.

[Laudanum]

Williaty ... here a good link.  It's for a stepped attenuator but would just as well for a rotary pot.   Paul J was discussing this with me a while back ...  http://www.goldpt.com/mods.html

Viktor ... I use an AH Tjoeb 99 CDP as one of my sources.  That unit is spec'd at 3.5 volts output.  It would have to be a "Hot" cd or track to reach that level but I could never listen that loud, not even close, even with my least effieicent phones.  I havent had a single problem with the Tjoeb as a source.  Let you ears be your guide.  As Grainger49 states and as Doc B has mentioned in a related thread ... hot source? ... that's what the volume knob (gain control / attenuator)  is for.

[williaty]

Part of the issue is almost certainly that I don't like loud things. I set the music just loud enough to clearly hear the quietest passages and then don't raise it any higher. Loud noises/music just give me a headache. So it's entirely possible that I'm using less of the pot because I'm trying to listen 10dB quieter than most people.

[Grainger49]

Try the series resistors I suggested.  If you can find them at Radio Shack that is a good thing.  I suggest getting an assortment pack.  Once you have zeroed in on the needed resistance then you can buy some $5 resistors that are better sounding.

[williaty]

I've got a packet of resistors left over from the Sweet Whispers build 10 years ago. I'll see if any of them come up useful.

[williaty]

OK, built the Speedball and installed it tonight. Voltages check out ok, but I have two different issues now.

1) The sound is more dry and trebly. Is that something from the Speedball or just how the recordings I'm listening to are actually mixed?

2) With no input, I have a strange very, very quiet rushing noise in the right channel only. This wasn't there before the speedball. Any ideas what this is?


Overall, I'm really very impressed with this amp.

[2wo]

1) let it run in for a while. Seems everything new I try sounds dry and trebly at first. If not you can always go back

2) if the noise goes away with the input shorted, or a source connected. don't worry about it...John