[BEGINNER] Modifications to Bugle2

[gattu marrudu]

Hello,
I am an audio DIY beginner, but being a longtime DIYer at several other things, I hope I can tackle this... bear with any possible absurd questions.

I bought a used Hagerman Bugle 2 for my Nagaoka MP-110 (5mV output) that I am driving with an Audio Nirvana 300B SET (8W) and AN 10' Classic drivers.

I mistakenly got an MC version of the Bugle2 (60dB). The output does not seem too high (still lower than my DAC) but I hear a lot of hum and buzz, plus the sound is very heavy on the bass. It is possible that the wrong gain is affecting RIAA equalization?

My main question on this forum is, can I make a simple modification to my Bugle2 to bring down the gain to my MM level, which I guess should be ~46dB?

Any other significant, simple component improvements that I can make on this board (gold-plated connectors seem the most obvious)? In the long run I'd like to build a tube phono stage from one of the designs on this forum, but until I learn enough about that I'd just like to enjoy my vinyl.



Thanks in advance for the help.
gm

 

[sgrossklass]

It is possible that the wrong gain is affecting RIAA equalization?

No, but the 100 ohm input impedance sure is! Keep in mind that MM cartridges are highly inductive and their impedance is in the kOhms by 1 kHz, reaching the tends of kOhms by the upper end of the audible range. You're losing a lot of treble if you're loading that with 100 ohms.

First of all, obtain the Bugle2 manual. Differences between the MM and MC values are detailed in the assembly guide as well as the schematic found therein, MC values being given in brackets.

The list of parts affected is rather short:
R23 (100R) - not populated
R1 (100R) - 332R
R3, R8 (392R) - 1.30k
Repeat on the other channel.

Note: I would suggest something a bit different.
R3 - leave at 392R, instead swap R4 for 3.92k (and should you need more gain, increase R4)
R23 - can be used to fit extra input capacitance as required by cartridge and existing cabling, 47 pF is probably a good idea for EMI considerations alone
U2 (LM4562) - (optional) try e.g. NJM2068, LM833 or NE5532(A) here instead, the LM4562 has too much current noise to be ideal for an MM cartridge (see here).

It's probably a good thing that you want to use it as an MM preamp, its noise performance would be well below par for MC.

 

[gattu marrudu]

Thanks a lot for the quick reply. I will order the parts and report back. This is a great holiday SUP (step-up project) for my soldering skills...

 

[sgrossklass]

PS:

Keep in mind that MM cartridges are highly inductive and their impedance is in the kOhms by 1 kHz, reaching the tends of kOhms by the upper end of the audible range. You're losing a lot of treble if you're loading that with 100 ohms.

... easily 30-40 dB, to be precise.

Note: I would suggest something a bit different.

Bad wording. I basically meant a modified modification, i.e. do swap out R1, R8 as indicated before.

The prominant hum and buzz may indicate further issues in your setup - like dirty cartridge or headshell contacts, incorrect wiring, some ground not hooked up, or similar.

 

[gattu marrudu]

I had a very strong <t-t-t-t-t-t-t-t> noise when I placed the pre near a computer monitor, it got much better but not disappeared completely as I moved it farther away. I have a small surface with lots of cables. Maybe shielding the enclosure will help?


My cart ground runs from the arm to the chassis (stock Thorens TD1-150). I will try rerouting it to the Bugle's ground post and see what happens. Some more hum happens when I land the cartridge on the record, that is mechanical stuff that I'll have to resolve separately.



gm

 

[sgrossklass]

I had a very strong <t-t-t-t-t-t-t-t> noise when I placed the pre near a computer monitor, it got much better but not disappeared completely as I moved it farther away. I have a small surface with lots of cables. Maybe shielding the enclosure will help?

It's a clear plastic case, isn't it? Definitely seems worth a try then. If nothing else, some Al foil with generous holes for the RCA jacks should help - make a small hole for the ground screw post.

My cart ground runs from the arm to the chassis (stock Thorens TD1-150). I will try rerouting it to the Bugle's ground post and see what happens.

I haven't looked up what that wiring looks like, but you definitely want entirely separate left channel, right channel and ground connections here. Tonearm ground to chassis basically isn't bad but then chassis ground also needs to make its way to the phonopre.

Some more hum happens when I land the cartridge on the record, that is mechanical stuff that I'll have to resolve separately.

That clearly shouldn't be happening in a subchassis player, at least with the transport screws loose. The dampers and motor run capacitor may have seen better days.

Is that the TP 13 tonearm you've got on there, complete with no antiskating? That would be a bit of a bummer.

 

[gattu marrudu]

I haven't looked up what that wiring looks like, but you definitely want entirely separate left channel, right channel and ground connections here. Tonearm ground to chassis basically isn't bad but then chassis ground also needs to make its way to the phonopre.

I misstated, I am actually running a wire from the chassis connection to the Bugle's ground.

That clearly shouldn't be happening in a subchassis player, at least with the transport screws loose. The dampers and motor run capacitor may have seen better days.

The TD-150 has no transport screws that I can see (the platter bounces freely). I could replace the dampers but I'll try not to be too invasive for the time being. However I took the opportunity to add some TT capacitor replacements in my order for the Bugle replacements.



I will also replace the ~50 year old signal cables and blackened contacts with RCA plugs and Belden 8402 cables. Same with the power cord. I want to do this in a new plinth, so I don't bore holes in the Thorens' pristine original chassis.



I polished and oiled the shaft, it may not be in pristine conditions, but not too bad either. It's the first one I see so I cannot judge.

Is that the TP 13 tonearm you've got on there, complete with no antiskating? That would be a bit of a bummer.

Yes, TP13 complete with no antiskating. I may attempt rewiring that at a later time, it may be a while before I can afford a better arm.