Feedback blew my phono stage?
Thanks all for your concern and contributions.....particularly Al and the Professor (Timeltel). I was hoping Al would have some theory behind this and the Professor's thoughts are most interesting. I had my own theory here:- The problem is structure-borne feedback as is mostly the case with turntables?It was in the act of trying some forms of 'isolation' that I 'blew' the phonostage. I am awaiting the arrival of Stillpoints Ultra Minis to really test whether my theory 'holds water'? My fear was......that it doesn't solve the problem and I am suddenly without any theories?! Moving the turntable may then be the only option although it is difficult for several reasons? I was just musing on the input I have received so far......one of my Vandersteen 2Wq subwoofers is directly behind the speaker under the turntable in question and is consequently right in the corner. Could this be the cause of the low-frequency excitation with the higher gain stage of the LOMCs? Alas....I must await the return of my preamp to test this theory? Thanks again gents....and any further thoughts are most welcome? Henry |
Another clue........ With the stylus not in contact with the vinyl........there is no feedback, which indicates to me that:- 1. It is not an air-borne problem 2. It is not EMI or RFI 3. It is not motor induced 4. It is not coming through the armpods 5. It is coming through the platter But when I change the mat from pigskin to rubber to copper.........there is no difference where I would expect some? The Raven is in exactly the same situation vis-a-vis speakers, walls and subwoofer.......yet is immune due to the Stillpoints/plinth/platter or motor locations? There is an answer somewhere.........? I hope? |
The Halcro DM10 went to my local technician who....when he opened it up FOUND THIS The Halcro designer in Adelaide said that the thick paint overlay was done to protect against patent infringement 'copying'....and also to prevent oxidisation? My local technician said it's probably more likely there to prevent anyone other than Halcro, from working on the circuit boards? In any case.....without the circuit diagram.....my local technician could not repair the unit so off to Adelaide the preamp went. Today the Halcro designer writes:- There were 3 faulty parts. Each input has a common mode choke to reduce noise. Both the phono stage chokes were damaged. There was also an electrolytic capacitor which had failed. Some of the other caps were also showing signs of aging (these parts are known to degrade over time). I took the liberty of replacing all of these caps as a preventative maintenance measure.Perhaps I need to tell him of the 'feedback problem' :-( And how do I ensure it doesn't happen again? Or could perhaps some faulty parts have caused the 'feedback' in the first place? |
I received the repaired DM10 back from Adelaide and re-inserted it in my system. I must admit that even before testing it.....I was 90% sure that the turntable 'ground' to one of the DM58 Monoblocks was indeed the problem? All the symptoms fitted including the 'coup de grace' when I placed the solid copper 1.8Kg Micro Seiki Cu180 'mat' under the TT-101 in direct contact with the metal base of the cage. Without understanding electrical theory........I assume the potential voltage difference was increased enormously by this action which explains the fact that when I turned on the preamp........there was an immediate 'explosion' (even though the volume was turned down) which 'blew the chokes and capacitor? I disconnected the 'ground' wire and placed the LOMC FR-7f cartridge and turned up the volume gradually. Although there was a low 'hum' as the volume increased.......there was no 'feedback loop'. I then connected the devilish 420STR, placed it on the vinyl and gradually increased the volume. No hum and no 'feedback loop' even when the volume was at maximum!? Problem solved.......and as so often occurs.......I was the problem!!? :-( |
Hi Al, I mentioned to 'Mr Halcro' that I had the 'ground' cable of my Victor TT, attached to one of the DM58 monoblock amps and could THAT cause the problem? It is possible that the grounding on the power amp may cause a voltage difference that could burn out the chokes. I would also be interested to hear if the feedback problem went away as well.Is his reply. What do you think? Regards Henry |
Thanks guys, I appreciate all your help and support during this harrowing process. Yes.....I'm lucky to live in Australia where Halcro was designed and built. The longest processes were transporting the preamp back and forth to Adelaide (850 miles).....and the cost of that exceeded the repair costs. Don't know what you would do if you lived in another country though?.... Now to listen with fresh ears to compare the sound to that of the beautiful valves I have become used to over the last 4 weeks? |
