Garrard 301 Bass feedback loop - how to break?

[Gerry_the_Merry]

Folks, I have heard this problem beset many TT set ups, but fortunately had never been till now, faced with it.

Recently after shifting residences, thanks to a firm push by JLS001 yesterday, I got my Garrard 301 up and running. It does 9o% of what my digital rig does, and there is a good bit more potential to be extracted.

It is set up as follows. A 40mm 7 kg sandstone slab cut out for the 301 is placed in a teak wood box, which is mounted on spikes, which are in turn on metal feet, which rests on another sandstone slab which is a 100kg slab, which is mounted on solid steel pillars, which in turn is on a teak wood rack, which is on the floor.

The speakers are on spikes which are on ebony feet.

The digital system has absolutely grain less sound, after various types of tweaks (the most important one being hardwood under the amp, as suggested in some of the RMAF videos.

When I power on the TT, there is no hum. When a i place a record, and the needle on the record, and turn up the volume a bit, and leave it there, without the record spinning, the trouble starts.

After about a second, a mild hum begins, which increases in volume in 2-3 seconds, till I am quickly forced to bring the volume down.

If I play a record and place the needle on the record, I cannot hear any difference between teh TT and digital, and the bass is identical in both. I tested late last night with a super mint dire straits LP communique, and cannot hear any extra unwanted bass on the TT.

IMO, the problem mentioned above is a mechanical problem, with a bass feedback from the floor being picked up by the rest of the physical rig and then teh arm and cart, which gradually gets amplified.

I don't want to put too much rubber and damp everything and lose the liveliness, as I am very happy with the sound as things stand now.

S my questions are -

- is this normal?
- when I play the TT, how come I have no extra bass? Why does it seem to come only with a stationary record?
- how to break this loop? Surely it will lead to some further cleaning up of any residual smearing.

Please suggest any remedies, and Thanks a lot in advance.

 

[kuruvilajacob]

I have some questions to ask you. Is this the first time you are using this TT? If you were using this TT earlier, were you getting extra bass in another set up in another room? When you keep the stylus on a stationary record and if you increase the volume, there is a chance of getting a magnetic feedback in some rooms and it may appear boomy and appear to give extra bass. Try moving your speakers farther from the TT and see if there is a difference.

 

[analogadikt]

Hallo GtM,

The problem that you are experiencing now is known as acoustic feedback. As suggested in the previous post move the speakers, especially the right speaker away from the turntable. It shall also help to have both the speakers in one line and the tt positioned behind that line. Is the problem intensity same with lid up and down? Kindly check and let us know.

Regards,

 

[Gerry_the_Merry]

Thanks for the above suggestions. Th speakers cannot be moved, as the position has been frozen in a way that produces the exact compromise between imaging intensity and soundstage width that I want, after many months of experimentation. But since you are implying that the tonearm / cart may be to close to the speaker, I can swap the position of the amp and TT. This will bring the arm roughly in the middle of the speakers, which have a 7 foot distance between them. Hence 3-3.5 feet should be enough, but let me check this out and post feedback.

It have used this TT for an year quite regularly, but was more focused on digital, which is now complete. So I don't remember if in my previous residence I had any problem of this kind or not. It was on a thinner sandstone plinth (16mm), and before that hardwood. The sound in this house is several levels better with teh digital, so the earlier sound I have forgotten totally.

Even here, or in the previous residence, there is no extra bass.

I just checked that with the lid on or off, the feedback comes.

The TT is slightly behind the line connecting the speakers, but I can move the rack back a bit further.

 

[Hiten]

Little confused with bass feedback coming from the floor even if record is not playing ?

Can you please do this. Do not turn on the TT put needle on record.

- Do you still get hum/bass ? If you do there may be a source of vibrations. It could be anything. Amplifier, transformer, CD player or just environment.
- if not the source would be the turntable itself. I mean motor or magnetic field etc.

am just guessing so could be wrong.
Regards.

 

[Gerry_the_Merry]

Hiten, just to be clear, if I switch on the TT or leave it off, there is no hum anywhere, even at full volume. It starts only when teh needle touches the record, when teh record is not spinning. It starts at low volume and then builds up. You would have seen this type of build up in many places when a mike and speaker come too close together. Something of that nature is happening, but as mentioned in the previous responses, it may be solved when I move the TT arm a bit further away from the speaker.

 

[Hiten]

ahh got it like microphone feedback its amplifying hum or vibrations.
Regards.

 

[Hiten]

- when I play the TT, how come I have no extra bass? Why does it seem to come only with a stationary record?

only explanation would be the hum/bass is in lower region than low fequencies of music content.
Regards.

 

[quad]

Do you have another phono pre to try?
just wondering if it is parasitic oscillation.

 

[mpw]

is there a cable TV ( set top ) nearby.. if so.. try switching it off.

mpw

 

[kuruvilajacob]

I still think it is magnetic feedback. Try placing the TT away from speaker and if the problem continues, try another phono. Also open up some window to ventilate magnetic feedback if any.

 

[reubensm]

Gerry, this might sound quite radical but have you noticed the same symptoms with a different cart on the same tonearm?

 

[cat's squirrel]

this seems like a classic case of feedback. Either from the speakers vibrating the floor, which vibrates the cabinet, slab, plinth and finally the turntable/cartridge/stylus, or more directly between loudspeaker and plinth/turntable/cartridge/stylus. The slow building of the resonance (used in its proper sense) will happen when the record is stationary, rather than when it is rotating is because the way feedback works.

The feedback will be at a certain frequency, determined by the thing that is vibrating most. Sound from the loudspeaker may be broad spectrum, but only a narrow band of frequencies will be excited in the vibrating structure. This builds slowly, depending on various factors, including damping.

Of the materials you mention that make up your turntable and support, none have very much damping, so vibrations will build over time until a resonance is formed. This occurs when the forcing frequency (from the loudspeaker) is the same as the natural (or fundamental) frequency of something in the structure that is vibrating.

Slow build suggests that some damping is present, but not enough to deal with the feedback effectively enough.

To remedy the situation, you need to add damping, so that the energy does not form a feedback loop. The structure you have effectively couples floor to plinth, so you need a damping layer to decouple this. If you don't, you may be suffering 'sound enhancement', although this is common with people who need 'exciting' sounds.

 

[Fantastic]

From the picture it looks like your TT is sitting directly on the stone plinth . Can you place a 4mm thick foam rubber spacer between the TT and stone slab. Typically like what some use on a Thorens TD124. You will need three or four pieces to do this. The foam rubber is used in automotive applications. It should take the weight if the size is correct. Maybe a 1"x1" piece at each location should do.
This is acoustic feedback. Just as an experiment , place the TT on the ground and see if it goes away or if the build up time changes. If it stays the same , most likely place to attack the problem is the TT plinth. The build up is volume level dependent ( position of volume control) I'm sure ......is it not ?

I don't see ANY decoupling anywhere between pointed legs and the turntable itself , in the picture. Did you look into that ?

 

[Hiten]

Last night I checked with my DD TT. With TT On and off there was slight hum but at extreme volume levels (3 o'clock). Interesting thing is I turned on the TT put a TV remote on TT and put stylus lightly touching it. There was hum at exact same volume level (3 o'clock) significantly almost double compared to earlier stage when stylus was not touching anything. I normally listen to at 9/10 o'clock position. So I think it is vibrations but at that volume level its ok.
Regards.

 

[greenhorn]

I had the same issues, no luck so far
http://www.hifivision.com/phono-turntables/45161-turntable-vinyl-queries.html

Moving the speakers have given me the best results so far. It's not eliminated, but at least reduced.

 

[Gerry_the_Merry]

Thanks a lot to all of you.

1) if I remember Lenz's law correctly, a moving coil in a magnetic field will produce current. Except that in this case perhaps the wrong magnetic field, caused by the speaker's magnet, is adding to the current in the cart.

2) hence this additional current, gets amplified etc. then feeds back through the speaker, and hence builds up. if I break this by de-coupling, the fact still remains that the original distortion is still in place, except it is no longer amplified in further loops. By original distortion i mean the impact of the speaker's magnetic field on the cart.

3) if I swap the amp and TT, then the arm being on the TT's right, the arm will now be close to the amp. The amp has a 1kva transformer. that will have its own magnet. I have no idea if the transformer is magnetically shielded, though it is compleetly sealed (Accuphase amp). So if it is not shielded, i will still get another unwanted magnetic field in which the cart's MC will move right?

Just want to get your thoughts before i do the heavy shifting to swap the amp and TT.

 

[Saket]

I had exactly the same problem and posted about it here: http://www.hifivision.com/phono-turntables/31168-huge-acoustic-feedback-tt.html

I found a resolution in a way, but due to some changes again in my set up, the feedback problem has come back. However, thankfully, only at high volumes, hence I can live with it most of the times.

 

[Hiten]

I doubt if speaker magnet would induce any magnetic field to a cartridge through panels and headshell etc. but I could be wrong. Instead of moving heavy stuff would suggest take a simple metal box type object (Faraday cage) with one end open and cover the cartridge to see if speaker or transformer is causing the hum.
Regards.
addition : by faraday cage I mean not mesh or perforated metal just plain metal box to stop magnetic field

 

[jls001]

3) if I swap the amp and TT, then the arm being on the TT's right, the arm will now be close to the amp. The amp has a 1kva transformer. that will have its own magnet. I have no idea if the transformer is magnetically shielded, though it is compleetly sealed (Accuphase amp). So if it is not shielded, i will still get another unwanted magnetic field in which the cart's MC will move right?

Before doing this swap, try moving the TT to the left, nearer to the amp. The Mac Mini gets displaced temporarily. This will increase the distance from the speaker's magnetic field to the cartridge by the inverse square of this distance (say, 7-8 inch?). In other words, the field strength of the speakers should decrease very substantially when moved away even that small distance since it follows the inverse square law.

If only you had a longer interconnect, you could place the TT on the floor on the carpet and play it from there to rule out magnetic field from the right speaker as prime suspect. You probably need a longer tonearm cable to be a bit more flexible with placements.

Try to, as far as practicable, run the tonearm cables farthest from the amp's power supply module. This is to avoid picking up another variety of noise - the electrical hum.

But to come back to the fact that the bassy noise comes on some moments after placing the cartridge on a stationary record, why should the noise come only after some time? Is there some sort of magnetic interaction that takes some seconds to reach a crescendo/peak? Even when the tonearm is resting in its stand, it is still subject to the same magnetic field from the speakers. In fact, strictly speaking, it will be subjected to a stronger field when it is at rest on the tonearm, since it is several inches closer to the speakers than when it is resting on the starting groove of a record. By the same reasoning, the bass noise should reduce substantially in the inner grooves.

Others suggested that this could be acoustic feedback. AFAIK, the OP said that the hum arise when nothing is playing. So can the cause be possibly acoustic in nature? I am thinking aloud here.