Garrard 301 Bass feedback loop - how to break?

[analogadikt]

GtM,

We first need to identify whether the problem is acoustical or electrical. Please try to carry out step by step checks as follows.

1) Power to all system components should be off to start with. Now, switch on power to the amp, select phono (switch on phono stage if you have stand alone one) and switch on the amp. If the hum starts now it is acoustic. You can push away the nearest speaker to the tt for a moment to check but it is very unlikely that the driver's magnetic field has any effect outside the box.

2)If all is well at the first stage then switch on power to the tt at the wall socket. after that flick the stop start lever and let the platter roll.

3) after this move the tonearm over the platter/record and again check by increasing volume. set down the stylus on the record.

Let us know exactly when the problem appears and goes away. Checks as above should enable us to pinpoint the nature of the problem although I believe that you have acoustic feedback and a sand tray under the turntable shall help contain that.

Regards,

 

[cat's squirrel]

the cause of the feedback is that some part of the turntable/support is vibrating. It is vibrating because it is being forced into vibration by the loudspeaker, either directly through the air, or through the floor. As the turntable is connected (coupled) directly to the floor, I suggest this is most likely to be the route of vibrations.

Whatever starts the vibrations (it may be hum) the stylus will pick this up. It will be amplified and the loudspeakers will sound a little louder. This sound is then picked up by the stylus, amplified, and fed to the speakers, which will sound a little louder, and change in frequency slightly towards the natural frequency of whatever is vibrating. This continues, until the sound grows, and we call it feedback. Note, the record does not have to be revolving for this to happen. The OP stated that feedback did not occur if the record was rotating, this is because the forcing frequency is changing with the music, and the resonance has not the time to build.

An illuminating experiment is to place the stylus on a stationary record, with amplifier on and 'speakers connected. Then arrange for a separate source of sound (a portable radio, for example) to play near to the record. It is possible to hear the radio through the 'speakers. This shows that the turntable/record are vibrating under the influence of the sounds from the external source, in this case the radio, and amplified, to be heard through the speakers. If this is the case, then there is something wrong (read poor damping) with the turntable/support, and probably position of the 'speakers.

 

[Thad E Ginathom]

Eliminate the physical coupling by picking up the deck, put it on a cushion, or on a small table within cable-reach. Use a worthless LP in case of accident, because it probaly will happen or just put the stylus on the platter with no platter.

If it is carried in the air, then putting your hand on the turntable when you can hear the feedback should at least alter it. I think?

Probably you tried all that anyway?

 

[Saket]

If you are certain of not damaging anything, the best experiment to carry out to know if it is the vibrations or the magnetic effect, just pick your deck from the table with your hands while playing. If the problem is related to the vibrations coming from speakers, the hum will stop soon; if it doesn't, you have the other problem.

Just my 2 cents!

Regards,
Saket

 

[shafic]

I also had problems ( Hum & Acoustic? feedback) sometimes back. Even i got the same opinions from the "experts". Eventually it turned out that the 'HUM' was due to an earthing problem in my house (thanks VT). The Acoustic feedback was not solved. My humble opinion is that, that particular TT was the culprit,as i did not face this problem with the other TTs. I fried two tweeters trying to solve the problem.
regds,
shafic

 

[Gerry_the_Merry]

Many interesting suggestions. Thursday is a holiday and I will read these carefully And eliminate this problem. Especially interesting, and novel, is the one about opening the curtains and ventilating...perhaps open windows may let in some light and reduce any gloom induced by the feedback. But cat squirrel's latest post is exactly how I perceive the problem.

If I solve the coupling issue, it still does not eliminate the original interference. Only that it won't get amplified in the loop. So will need to determine that also.

Picking up the TT and its 20kg weight is impossible but I can introduce some rubber cushioning under the stone cutout or between the TT footers and the stone surface. Don't want too much rubber which can reduce the liveliness.

 

[Thad E Ginathom]

20kg weight!

 

[jls001]

If vibration is prime suspect, "Vibrometer" app or "Vibration Monitoring" app (on Android) will be helpful to check vibrations. If you use an iDevice, I believe there are equivalents.

 

[jls001]

This long pending problem of GtM's G301 was solved today. Kuruvillajacob, analogadikt, cat's squirrel, etc - you guys are absolutely right in your diagnosis of the problem. It was indeed an acoustic feedback coming from the speakers. GtM uses a heavy and large acrylic cover for his 301. He normally keeps the lid of the TT closed when playing. As the volume knob is turned up and as more air is pushed out by the speakers, it induces a vibration on the large surface area of the acrylic cover. Which in turn causes spurious resonance on the TT, causing an irritating and loud hum.

The solution was trivial - keep the lid opened when playing. No hum at any volume. It was a serendipitous discovery, as I have struggled and resolved electrically-induced hums, but I had never faced acoustically-induced hum. I was standing between the TT and the right speaker when GtM suddenly increased the volume. I could feel the phone in my hip pocket literally vibrating!

PS: Vibrometer app (on Android phones) was really useful in measuring the vibration on different parts of the TT top plate, plinth, equipment rack, and bare floor. Highly recommended.

 

[kuruvilajacob]

It is better to have dust cover which can be removed while playing the TT. I have had many chaps who have purchased Turntables from me complaint about this acoustic feedback and most of the time, the repositioning of the systems solves the problem. I am right now in Bangalore and I had visited a FM who had purchased a TT from me and has this problem while increasing the volume. I have advised him to keep the speakers further away to solve the feedback problem.

 

[Thad E Ginathom]

This long pending problem of GtM's G301 was solved today. Kuruvillajacob, analogadikt, cat's squirrel, etc - you guys are absolutely right in your diagnosis of the problem. It was indeed an acoustic feedback coming from the speakers. GtM uses a heavy and large acrylic cover for his 301. He normally keeps the lid of the TT closed when playing. As the volume knob is turned up and as more air is pushed out by the speakers, it induces a vibration on the large surface area of the acrylic cover. Which in turn causes spurious resonance on the TT, causing an irritating and loud hum.

The solution was trivial - keep the lid opened when playing. No hum at any volume. It was a serendipitous discovery, as I have struggled and resolved electrically-induced hums, but I had never faced acoustically-induced hum. I was standing between the TT and the right speaker when GtM suddenly increased the volume. I could feel the phone in my hip pocket literally vibrating!

PS: Vibrometer app (on Android phones) was really useful in measuring the vibration on different parts of the TT top plate, plinth, equipment rack, and bare floor. Highly recommended.

Amazing! And how satisfying to find a simple answer to the problem. Congratulations, well done!

:clapping:

~

 

[Thad E Ginathom]

Sometimes the simplest answers can be the hardest to find.

 

[mpw]

Or..the complex things In life are sometimes the most simple.

Great job guys !!

Mpw