[jaevans]

Greetings all,

  I built the Bugle which seems to be working fine for me except for the fact that I seem to be getting a lot of clipping on the output.  I built a T-attenuator pad for the output (now my sound card likes the signal levels it sees) but I am still getting clipping according to the waveforms tracked in Audacity.

  I tried building an attenuator for the Bugle input but I get a lot of noise and hum introduced into the signal (I may try this approach with a small metal box for shielding).  Would a turntable attenuator be the way to go or can I adjust the opamp gain resistors on the input opamp to drop the gain some?  At first I thought my attenuator design was clipping the signal but after three iterations of T-pads, I feel the clipping may be occurring in the Bugle.  The Shure cartridge specs are well under the overload specs on the Bugle, but from the start the output to my soundcard always seemed way too high.  Same symptom on the audio chipset that came with the motherboard as well.  Any ideas?

  Setup:
     AR-XB turntable
     Shure M91ED cartridge with N91ED needle
     Bugle preamp with external Hagtech power supply
     ES1371 Creative Labs soundcard (occurs with motherboard sound chipset as well)

Thanks much - john

[GRD]

I'm afraid I'm going to show my ignorance, but maybe someone more experienced will chime in.

My guess is the Bugle is likely doing OK, and not clipping with your cartridge input.  Jim has a tool on his website for changing the gain - the recommended resistor and cap values needed.  So you could reduce the gain if you wanted, but that may not help if the problem is elsewhere. 

Don't know if there could be a problem with the padding and input resistance of your board (presenting too low a resistance to the bugle output).  But I also looked at the output impedance of the Bugle and it's pretty low - around 300 ohms.  So you should be able to pad the output pretty easily.  What value resistors did you use to pad the output?

That's all that I can come up with.  I'll turn it over to the experts.

[jaevans]

What prompted all this was the fact that the output from the Bugle, driven by my turntable, into the soundcard was just way too high.  I could barely get the line in volume down to a manageable level within Audacity.  I was basically on the last tick of the slide control before the minimum/off setting.

So I worked on a couple of attenuator pads to reduce that volume.  All of them clipped.  The most recent T-pad with 22K ohm input and output resistors and 33K ohm shunt seemed to do the best in terms of actual volume range into the soundcard (the pad was connected between the Bugle output and the line input on the soundcard).  The pad's design was based upon an input and output impedance of about 47K ohms. 

I couldn't locate the design tool for determining the Bugle gain settings, but the circuit looks simple enough with respect to gain adjustment - the gain of op amps is typically determined by a voltage divider - in this case, the resistor pair is such that the gain for the first stage is set at 11.  I'll need to research this some more before "playing" with the gain to determine the impact on other aspects of the Bugle - if this is the correct line of thought then I will pursue it, otherwise, it is back to the drawing board - perhaps trying a different preamp to see how the volume is into the soundcard.

john

[GRD]

John,

Here's the link, and it is sort of hidden in the Bugle section.  Scroll down to "calculate bugle values".  Hope this works for you.

http://www.hagtech.com/equalization.html

Regards,

Grant

[jaevans]

Thanks much

  That does give me another option if I exhaust every other avenue.  Just for a sanity check I opened up the preamp and verified that I had the right resistors installed in each and every spot and measured them all, though my meter seems to be reading a bit on the low side - at least I'm not an order of magnitude off anywhere.

  Next thing to do will be to replace all three of the OPA2134 op amps - I plan to do this to eliminate any issues at all with the Bugle and to reassure myself that an assembly blunder didn't damage them - I was really careful during assembly - the sockets are oriented properly - but I then stupidly installed them all in backwards in spite of the socket orientation - call it a brain fart due to age.  Anyway, it blew the tiny fuse in the power supply at smoke test.  I discovered my error and corrected their orientation and all seemed to work fine after that.  I want to make sure that I didn't damage the op amps in such a way that could cause any clipping or distortion.

  Film at eleven
  John

[GRD]

John,

Tell us how it turns out. 

Grant

[jaevans]

Update !!!

It seems I was focusing too much on the 5-10 db range of attenuation - I built a 20 db attenuator T-pad with 47K input and output impedance to insert between the Bugle and my soundcard and now there is no more clipping of loud sections of music from LPs.  I think the audio may be a little low at 20 db so I am about to try a 16 db T-pad to see how that is.  At any rate, I am not seeing any clipping at all now.  I may even look into building a step attenuator so I can switch in a variety of values based upon the LP I am ripping.

Joy, joy !!!   

[GRD]

Excellent!!

Thanks for posting your findings.