What does a scratch built VTA ST-70 sound like when it clips?

kaner wrote:I had a similar issue with a guitar amp.  In my case it was a cold solder joint.  The vibration from the speaker was revealing the issue.  Are your speakers close to the amp?  Also check your tube sockets to make sure that you have tight pin connections.  The vibrations can cause the tube to jostle in its socket.

corndog71 wrote:Are you sure it's the amp and not the preamp you're overdriving?

Downhome Upstate wrote:

corndog71 wrote:Are you sure it's the amp and not the preamp you're overdriving?

Hi Rob,

No pre in the circuit. It's an Oppo 93; I'm using the digital volume adjustment.  

I'll insert the Extended Foreplay III in the setup and then hear what happens.

Best,

Mike

sKiZo wrote:I clip my ST120 regularly ... sounds like a bad haircut, short on the sides and prickly in the middle ... much like a Brian Bosworth mullet ... it'll lose separation and get fuzzy up top mostly. None of the great crunching noise of doom my McIntosh MC2125 would be making at the same SPLs.

Thing is, I don't much mind it as the amp doesn't slam into a clip like solid state ... any distortion builds gradually, and it's on peak transients only, so more times than not, it's not all that annoying, and certainly not enough to start popping cones. I'd go so far as to say most clips go completely unnoticed.

If yours is still using the "classic" power transformer, it could also be the iron is tired. The original PT is borderline at best and doesn't get better with age. Browning out the peaks can really change what you hear.

kaner wrote:Can you temporarily use long speaker wires to move the speakers or amp into a different room?  That way you can take the vibration issue off the table.

tubes4hifi wrote:I'd measure the output of the CD player, the input to the amp shouldn't be more than about 2v p-p, or 700mv rms, to get full power without clipping.
Unless you're running at full volume into difficult speakers and playing mostly sub 50Hz notes, you shouldn't have any clipping.
Turn it up to 11 . . . .

sKiZo wrote:I clip my ST120 regularly ... sounds like a bad haircut, short on the sides and prickly in the middle ... much like a Brian Bosworth mullet ... it'll lose separation and get fuzzy up top mostly. None of the great crunching noise of doom my McIntosh MC2125 would be making at the same SPLs.

Thing is, I don't much mind it as the amp doesn't slam into a clip like solid state ... any distortion builds gradually, and it's on peak transients only, so more times than not, it's not all that annoying, and certainly not enough to start popping cones. I'd go so far as to say most clips go completely unnoticed.

If yours is still using the "classic" power transformer, it could also be the iron is tired. The original PT is borderline at best and doesn't get better with age. Browning out the peaks can really change what you hear.

tubes4hifi wrote:I'd measure the output of the CD player, the input to the amp shouldn't be more than about 2v p-p, or 700mv rms, to get full power without clipping.
Unless you're running at full volume into difficult speakers and playing mostly sub 50Hz notes, you shouldn't have any clipping.
Turn it up to 11 . . . .

Downhome Upstate wrote:

tubes4hifi wrote:I'd measure the output of the CD player, the input to the amp shouldn't be more than about 2v p-p, or 700mv rms, to get full power without clipping.
Unless you're running at full volume into difficult speakers and playing mostly sub 50Hz notes, you shouldn't have any clipping.
Turn it up to 11 . . . .

On my scope, the signal from the CD player is about 0.2 v p-p at the point where the harsh artifacts appear.  Any thoughts about what to look for next?

deepee99 wrote:

Downhome Upstate wrote:

tubes4hifi wrote:I'd measure the output of the CD player, the input to the amp shouldn't be more than about 2v p-p, or 700mv rms, to get full power without clipping.
Unless you're running at full volume into difficult speakers and playing mostly sub 50Hz notes, you shouldn't have any clipping.
Turn it up to 11 . . . .

On my scope, the signal from the CD player is about 0.2 v p-p at the point where the harsh artifacts appear.  Any thoughts about what to look for next?

This will probably get me flamed, but try considering your interconnects, or at least their contact points. Start with a little Deoxit on the cable ends and RCA connectors. I'm not recommending $10k 1-meter cables which are BS, but maybe you've got some resistance that with enough juice finally gives up, starts passing the demanded current, and then everything goes pear-shaped.
Just a thought.

Downhome Upstate wrote:

deepee99 wrote:

Downhome Upstate wrote:

tubes4hifi wrote:I'd measure the output of the CD player, the input to the amp shouldn't be more than about 2v p-p, or 700mv rms, to get full power without clipping.
Unless you're running at full volume into difficult speakers and playing mostly sub 50Hz notes, you shouldn't have any clipping.
Turn it up to 11 . . . .

On my scope, the signal from the CD player is about 0.2 v p-p at the point where the harsh artifacts appear.  Any thoughts about what to look for next?

This will probably get me flamed, but try considering your interconnects, or at least their contact points. Start with a little Deoxit on the cable ends and RCA connectors. I'm not recommending $10k 1-meter cables which are BS, but maybe you've got some resistance that with enough juice finally gives up, starts passing the demanded current, and then everything goes pear-shaped.
Just a thought.

Thanks for the idea, but I don't think that's it. They are simple but effective stuff: 1 meter Belden 89259 coax with Canare RCAs terminated by Blue Jeans Cable. They have no oxidation at all, and should transfer the signal just fine. Even if capacitance was an issue, they only add about 19 pf/ft.

audiobill wrote:Could your CD player be at fault?

Try one of your video disc players and see if a difference.

tubes4hifi wrote:I can't think of any possible way that 200mv p-p signal would cause the amp to clip.   It should take nearly 10X that level to cause any clipping.
Do you have a test CD or sig gen that you could set to that output level and then check the output side of each capacitor?

tubes4hifi wrote:I can't think of any possible way that 200mv p-p signal would cause the amp to clip.   It should take nearly 10X that level to cause any clipping.
Do you have a test CD or sig gen that you could set to that output level and then check the output side of each capacitor?

Downhome Upstate wrote:My scratch built ST-70 has all new parts (including the VTA noval driver board) except for the original cloth lead output iron. When I drive it hard, the signal begins to break up, producing static, cracks and pops. That doesn't seem like garden-variety clipping behavior. Any idea what this might be?

Thanks in advance for any suggestions and ideas.

Best regards,

Mike