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Dr. Know is our resident Technical guru, responsible for the free Reference Library, Technical Info, Data Sheets and the Audio Clinic. Contact the Audio Clinic at 
Dr. Know aims to reply individually to your sound system queries sent to his Audio Clinic. No appointments necessary. Some queries may be published. Radio mic queries are dealt with in the Radio Mic Help File Vinyl Turntable queries are dealt with in the Turntable Clinic

This site has gained an award from the Stagelink Online Directory resource. They awarded a "Recommended" status for Dr. Know's "excellent Reference Library" from 2004 - 2015. 

Your Questions about audio equipment:

1. Humming cam-corder mic
2. Identifying antique microphones
3. Seck mixer support
4. Multi-room home audio
5. How to use a compressor / limiter
6. Spare drivers for Yamaha NS10MS monitors
7. Where can I get a new turntable drive belt?
8. Where can I get user manuals?
9. How to clean clogged microphone grilles
10. Turntable tuning tips
11. Why is my speakers humming when I turn up the amplifier volume?
12. My Revox B77 record levels fluctuate
13. Black, sticky, oily goo in my tape recorder where a drive belt used to be

Q1 I recently purchased a ATR97 Microphone for use with a Sony 80X Mini cam. The microphone works fine but I am getting a constant humming noise that detracts from the noise I wish to record. Because I have to record from several locations there is a 50m length of cable from the microphone to the cam corder. Most of this is usually wound up around a cable reel. I have tried earthing the microphone cable both at the microphone and the cam corder ends but it only makes the humming worse. I cannot use the built in microphone of the cam corder because the nearby road traffic noise is distracting. As I record for hours at a time I use a mains power connection for the cam corder. Any suggestions? Many thanks in anticipation. G.D.

Dr Know' reply: You are almost certainly picking up hum due to the extension cable being used. What type of cable are you using for the microphone? For a 50M  extension you MUST use screened cable and preferably balanced (two core + screen). In order to get the advantage of balanced cable you will need a small transformer at each end. These will convert the ATR97's unbalanced mini jack to balanced XLR and at the other end, back down to unbalanced for the camcorder mini jack. These in-line transformers will cost £18.17 + VAT each with the appropriate plugs fitted. If your extension is not screened or not balanced, then you will need to replace it. A basic balanced mic cable (50M) will cost £36.31 + VAT. A high quality balanced mic cable (50M) will cost £55.81 + VAT. 

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Q2 I like your web site, which I've visited a couple of times now. I do a bit of amateur recording as a hobby but I've never seen a microphone like the one Jack McIvor uses in "Stiff". Can you tell me what type it is and is it an antique? Yours faithfully, M. Duffy (Scotland)

Dr Know's reply: Thank you for your comments and we're glad you like our site. The big old mic in the photo is actually a theatrical prop which has a cunning space inside to hold a slim condenser such as AKG C451, C460, SE300, Audio-Technica PRO37R, ATM33a, etc giving quality performance with retro looks.
However for "Stiff" we only used it as a prop since all the cast were wearing ATM73 head mics. A pre-programmed digital echo and band pass filter was added to give the effect of an old "tannoy" system.
[added Jan 2008, see our expanding range of original classic and vintage microphones for hire or sale.]   

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Q3 We are a company located in Austria, specialising in repairing professional audio devices. Recently we got a Seck mixing console - unfortunately without any schematics. It would be great, if you have some drawings - or information where to get them. I'd like to thank you for your efforts, with best regards.      M.M.

Dr. Know's reply: As you probably know, Seck were taken over many years ago by Soundcraft and much of the Seck design went into the Spirit series consoles. Unfortunately, Soundcraft don't support Seck products and we are not aware of any specialist Seck support companies.
We have limited tech info and compatible parts available. We have two types of new replacement power supplies available from stock. We often have  secondhand stripped down parts available. More detailed information and prices can be found on our data sheet 
I have now added some schematic and circuit drawing links on this data sheet for all the people who keep asking about Seck mixers. I hope this helps you. Let us know how you get on.   

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Q4 I read your web site with interest and wondered if you could help me with a query I have regarding my home audio set-up? I would like to install pairs of speakers in four different rooms and am looking for a simple (and cheap!) way of controlling volume in each room independently. Do I need to buy a separate amplifier for each room or is there a product on the market which will do this for me? Any help or guidance would be much appreciated! Many thanks,  S. J.

Dr. Know's reply: You are obviously someone who either loves his music or can't be without the radio. There are simple switching devices around £20.00 that can switch between four pairs of speakers from one amplifier but these do not vary the volume and you will need to be careful about the total load impedance presented to the amplifier. You will need to check the impedance of each set of speakers (will they all be the same or will they be different pairs?) and work out how to combine them safely. As a rule of thumb, most (but not all) domestic speakers are 8 Ohms impedance each. Most modern amplifiers will be fine driving a total load of 4 Ohms ie two pairs wired in parallel. If you want to control some of the volumes separately and/or run all four rooms simultaneously, you will need either a four channel power amplifier (eg Yamaha XM4220) or an extra stereo amplifier (eg Denon PMA250SE), fed from the pre-amp output of your main amp. This will allow you to run two pairs of speakers from each amp so you will be able to vary the volume in two stereo zones. If you really want to vary the volume in each of the four rooms separately, you will need a separate amplifier for each room ie four stereo amplifiers or a six channel power amplifier plus your existing amp (eg Yamaha XM6150).  

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Q5 Please can you help me I have a Drawmer DL221 compressor and it is my first time to use a compressor and I cannot get a good vocal sound out of it could you give me some information on settings because I have not got a clue. yours R.

Dr. Know's reply: Try this out first with a music source such as CD or MD or sampler.
Set the threshold and gain to zero. Set the other three controls to the left. Set the meter switch to VU and the function switch to bypass. Sound should be passing through, unaffected.
Now switch the bypass to Norm. If everything is wired up right you should hear things about the same.
Threshold determines the level at which the compressor starts to affect the sound. So the higher you set this, the less effect. the lower you set this, the more effect.
Ratio sets the proportion of sound compressed. 1:1 is no compression, 3:1 or 4:1 is usually a good vocal setting although light, breathy vocals will need a higher level. Infinity:1 is a maximum "brick wall" limiter.
The Attack and Release determine how quickly the compressor acts and de-activates on the sound, so you can experiment with these to balance leading edge dynamics and decays with compression or limiting control. These controls can give you some artistic control of the sound, making the compression sound more or less natural/seamless.
The Gain control is just a way of compensating for any loss of signal level caused by the compression or limiting. It operates after the compressor circuit.
Using the Bypass switch will allow you to compare levels against the dry sound.
The LED meter shows the amount of gain reduction (GR) or you can switch it to VU to see the actual level passing through.
If you use compression over a stereo source, switch the Stereo link. This makes both channels trigger simultaneously.  

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Q6 Hello,  First of all your site is great, it helped me a lot. I would like to order a NS10M part that is L.F. JA1801... Regards K. C.

Dr. Know's reply: No problem. Even though Yamaha, in their infinite wisdom, have discontinued the ubiquitous NS10MS, we have spare bass drivers, tweeters and crossover networks available @ £** each by mail / email / telephone order. Check out the full specs on our NS10MS data sheet.  

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Q7 Where can I find a new drive belt for my beloved old Dual record deck?    C.B.

Dr. Know's reply: We have a large range of replacement drive belts available @ £** each by mail / email / telephone order. See the chart and measurement guide on our Drive Belt Data Sheet    top

Q8 What would be the cost' Canadian ' for the operational manual for the Yamaha EMX 150 mixer , I just purchased one at a used item sale.  J.W.

Dr. Know's reply: Thank you for your enquiry. You can download a copy for free.
Use the link from our Technical Reference Library:
(Here you will find free links for most manufacturers' user manuals)

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Q9 Question is, do you have any sprays / fluids or services for cleaning clogged up microphone heads of the round mesh variety? Or can you point us in the right direction? Yours sincerely S.B.

Dr. Know's reply: Remove the grille from the microphone and wash with hot water and detergent, using an old toothbrush or nail brush. If the grille is very clogged up, leave it to soak a while in the soapy water. If it is rusted, scrub all the rust off with a small wire brush and then seal the bare metal with a coating of enamel paint or plastic sealant spray. Take care not to get the spray on the foam inside the head. If they are really bad, it will probably be easiest and safest to just replace them.  

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Q10 I have an old vinyl player which I am hoping to use to digitize my old LPs. What servicing tips would you recommend. Regards, G.

Dr. Know's reply: Check out my range of turntable tips and FAQs at my Turntable Clinic:   

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Q11. there is some buzzing interference which increases in loudness as the amplifier (Quad) volume is turned up. Any ideas? N.C.

Dr. Know's reply: Your buzz is quite possibly an earthing issue, meaning that the signal cable is not properly screened against interference. Alternatively it could be picking up mains interference by induction from a strong electro-magnetic field (EMF). Here are the main points to check:

1. Check that the four cartridge wires are correctly orientated according to colour.
2. Using a continuity meter, check that the earth wire (beside the phono lead) is well connected to the tonearm tube.
3. Make sure that this earth wire is well connected to the special turntable earth point on the amplifier.
4. Make sure that the amplifier is properly earthed to its mains plug.
5. Check that the earth/screen is well connected on each of the phono connector plugs.

Check that the phono lead is not routed next to any mains cables or power supplies.
Check that there are no mains leads or power supplies sitting close to the rear of the amplifier.    

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Q12. I managed to get fixed up with a Revox B77 for a very reasonable price. It also included the nab reels and 18  x 10.5 inch reels with tape (2 unopened). The only fault is that it does not record. There is a strong signal going into it as the VU meters bounce in time to the input but the quality of the recording is very poor (scratchy sounding with drop outs in sound etc). I cleaned and demagnetised the record head but it made little difference. Perhaps the head itself is at fault.

Dr. Know's reply: Whilst itís hard to diagnose 100% without seeing the actual machine, but assuming all heads are clean and demagnetised properly Ė as you say Ė and that it does actually record and playback.

When the recording is replayed and the levels are a bit low but stable, the likely causes are:

  1. Head alignment needs calibrating (all three heads)
  2. Replay amplifiers need calibrating
  3. Record amplifiers and Bias & EQ needs calibrating
  4. Do you have any pre-recorded tape or test tape to check the replay with?
  5. The head(s) may be worn out or damaged and may need replacing (rare)

When the recording is replayed and the levels fluctuate the likely causes are:

  1. Input amp card may need presets cleaning or replacing.
  2. Output amp card may need presets cleaning or replacing.
  3. Edge connectors of record and replay amp cards may need cleaning.
  4. Record switches and source selector switches may need cleaning
  5. Tape speed switch contacts may need cleaning

All of these procedures should only be carried out by a suitably qualified service engineer with appropriate test equipment.    

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Q13. I've  had the tape recorder a long time and not used it that much.  A few years ago it was switched it on to record something quickly and made a twanging noise, by which was assumed that the drive belt had snapped. To get to the nitty gritty; I managed to find the way to take the top off the recorder, expecting to find the space full of exposed works and a substantial drive belt, a big O ring in fact, lying amongst the workings, but no. There must be plenty of workings though all shielded by a metal plate topping with the drive belt pulleys and what remained of the belt on top of that.   I saw a black blob of something by one of the pulleys and removed it only to find it clinging to my fingers like that special soap you can buy in a joke shop that makes your hands all oily black.  It was very hard to remove and had to be scrubbed off.  Then I realised that the expected substantial drive belt was very slender and ran between two smallish pulleys, and that something has made it collapse onto the metal in a sort of mush, so you've got this line lying here and round the two pulleys.  Judging by how hard it has been to scrub off hands, it would be the very devil to scrape off the metal topping and can even imagine that the pulley wheels would have to be replaced.
So I'm assuming that the black semi liquid rubber will have to be carefully cleaned off by some special chemical to have any hope of fitting another drive belt. Please could you give the benefit of your knowledge so I might then be able to order a drive belt. Rgds, L V

Dr. Know's reply: Sometimes old belts dry out and just break. Other times, they melt and go all gooey, depending on the climate in which the unit has been stored. The best way to clean the molten rubber off the drive path is with a solvent cleaner such as isopropanol alcohol (IPA) or methylated spirit or nail polish remover. Try to avoid scraping the old rubber off with a blade in case you scratch or damage the drive pulleys or flywheel.

We do not sell drive belts for specific machines and we donít have any previous sales recorded for your model, however, we do sell generic replacement belts by length. Before ordering, please measure your drive path and follow the fitting guidelines at   

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Customer Quotes: 

"Hi,Thank you again for the great service and dispatch and very quick delivery of BELT9 reference order 164. Fitted it and BELT9 as per your listing is the right size for the NAD6040A  tape drive, it now works perfectly. Many thanks" Col (24/03/2023)

"Please accept my thanks for the two belts for my Dual 505/1, and for all the effort you put into the educational videos." Peter (23/03/2023)

"1 x belt T705 Marantz DVD player. Belt spot on and did the trick perfectly, 'belting' it out again, thank you." Nigel C. (20/10/2022)

"Thanks for the belt, I just found time to fit it to the CD player & it has restored it to working condition. For your reference, the CD player is a Cambridge Audio CD5 ( Audio_CD5_426.php ) and the symptom that this belt ( BELT61L ) has cured was that the tray would pull in but the CD lens mechanism would not raise up into position in order to initiate the reading of the disc. This has saved my partner from scrapping the machine & buying a new one! Thanks again," Tom (22/09/2022)

"Dear GB Audio -- Many thanks for your extremely prompt service. I ordered the replacement drive belt quite late in the day yesterday and it arrived this morning. Less than 24 hours later it's all up and running like new. picture & video attached" N Pemberton (28/07/2022)

"Dear Sir or Madam, Radiomobile [106S] unit model number as originally fitted to 1970s Rolls-Royce Silver Shadow. Repair was successful using your BELT12 308 x 6 mm. Many thanks. Best regards" H Drees (02/05/2022)

"Hi, Just a quick note to say thanks for your assistance in selecting the correct belts for my old cassette player. The belts fit perfectly and the old machine is now working as good as new. Great customer service. Regards" Ant (27/04/2022)

"Hi, Thanks for the very quick delivery of the BELT1295 I would like to confirm that this belt works perfectly as the replacement capstan drive belt in my Philips N7300 Reel to Reel Tape Recorder.  The Philips N7150 is the same machine but lower spec.  It has exactly the same drive mechanism so the belt will also be suitable for that model too. One again great service a great belt. Regards," Kevin. (25/03/2022)

"Thanks for this info, I had tried cleaning fluid unsuccessfully; but your comment made me try again.  I made a really determined effort to get into the switch and SUCCESS! Thanks again, all working like new again. Regards, C Kennedy (08/12/12)

"To thank you for the excellent turnaround on the item I purchased on Tuesday January 3rd. - an item which I ordered on your advice.
It was delivered on Monday 9th. which, considering that Friday 6th was a national holiday here in Spain was sooner than I expected.
The item is working perfectly. Many thanks". E Robinson (11/01/2012)

"Dr Know, Thank you for your time and for your wise advices". Gauthier (15/11/2011)

"That's brilliant! Thank you so much for the excent and speedy service. Best wishes," Lance. (03/09/2011)

"Dr. Know, I wanted to send you a note to compliment you and also say thanks for your extremely concise and helpful website information regarding production wireless equipment issues. It's quite obvious that you're "TOP SHELF" !  Was just surfing the web as I'm doing some planning for The Royal Wedding and came across your site... well done. If I may ever be of assistance to you for production wireless queries within the United States please feel free to give me call as I'm most happy to return the gestures you've extended to the professional wireless user community. Thanks Again ! KP" (Senior RF Engineer, National Broadcasting Company, News Field Operations, New York, USA 07/03/2011)

"Hello my name is J.K. I have found your website to be a valuable resource for wireless headphones and audio products in general. Therefore I would like to place a link on our website so that our customers can use your website as a resource".

"Dear Dr Know, Many thanks for your prompt and reassuring reply. Best wishes, B Bensley" (05/02/2010)

"I would just like to say that when I did a uk google search (drive belt), your site was the easiest to use and understand, with good, clear insructions. I especially liked the link to Pi, it made me smile". Best wishes, Ben UK (26/03/2009)

"hi, just bumped into your web site (through a search on some microphone specs i think) and its really nice. if more people actually put up some useful information on their sites then the web would be a better place..."  a.f. brussels


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