tpg Posted November 12, 2005 Share Posted November 12, 2005 No matter what I do, I have a hum or a buzz or a hiss when I connect my Forte Audio 1A to anything (and I mean anything). When unhooked, it is dead silent. I've tried my computer (which is the best, by far), my CD-player (which has been a REAL chore), and my portable CDP (which hisses like crazy and continually distorts everything). My luck hasn't been running well with this amp, and I am getting frustrated enough to sell it (typically this is what happens to me). I built a digital potentiometer that somehow has a ground loop even though it probably shouldn't. The right channel ALWAYS buzzes, and quite loudly. It is very distracting. The left channel has a slight hum that is inaudible unless right up next to the speaker. It really took some hooking up to get my CD-player in the chain. Apparently it has a really strong output, as it clipped my digital pot and made it malfunction (even when set to -20dB on the fader). My computer produces a very mild buzz, but some hiss that is slightly audible at listening position. But, it is really far away and not convenient for hookup. The sad part is that I hooked my speakers back up to my receiver and started listening to music... and it sounds just as good as the Forte when at its best (so far anyway- I've not been able to get its best out). The thing is, my receiver is so much easier to hook up. And, EVERYTHING WORKS! I've never had any trouble from it- no hisses, buzzes, hums, nothing. If hum and other problems come with real high end audio components, I think I am going to go back to the "ignorant" side. Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted November 13, 2005 Share Posted November 13, 2005 tpg: Do you have a variable output on your CDP, where you can control the volume with the remote (if supplied)? It might be helpful to simplify the system as much as possible, and maybe bypassing everything else will help point to some problem areas. This might also be associated with ground loop issues, as well as poor ground connections on the input to the amplifier. Do the ground barrells on the interconnect cables fit securely on the RCA jacks on the amplifier? This might sound like a strange question, but do you have any dimmer switches in your house, or other appliances, such as a portable fan, that are controlled by a variable control (sort of like a volume control)? Erik Quote Link to comment Share on other sites More sharing options...
tpg Posted November 13, 2005 Author Share Posted November 13, 2005 My CDP does have a variable output, but it only attenuates 20dB, which is not quite enough. It is still horrendously loud at -20. I have previously tried the -20dB setting with a resistor divider, but it hummed... I am a little sad about the digital potentiometer. It really should have worked. I tested it with my PC speakers (Klipsch ProMedia) and it worked wonderfully without any hum, hiss, or buzz. I hooked it up to my CDP and immediately noted a horrible channel imbalance (one channel was much quieter). I have heard that the DS1802 (the digital pot I used) can easily be clipped by some CD-players (apparently mine). This was even on the -20dB setting. The digital pot is 45k with push-button volume control. I had to add about 30k in resistors before the pot so it wouldn't clip. I tried various configurations, layouts, and cables, but all hummed or buzzed at variable levels. The right channel seems to buzz more than the left (though if I switch the cable leads around, the left channel then becomes the major-buzzer, so it is not an amp issue). I find it funny why this amp would be so much trouble. The digital pot didn't hum or anything when connected from my PC to my PC speakers. And, I've previously used the same RCA cables that I used to go from the digital pot to the amp to go from my CDP to receiver. So they shouldn't be the issue either. I am not sure if we have any dimmer switches or anything like that laying around unused... I'll check around tomorrow. Something about this amp just doesn't agree with me for some reason. It leaves me continually fearing for the safety of my speakers. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted November 13, 2005 Share Posted November 13, 2005 Yea I'm not sure what the Forte Audio 1A really.... is an amplifier? Would you happen to have some specifications on it? It sounds like it might be too sensitive so your picking up any hint of noise from any source you attach. Get us the sensitivity specification. Just found your other post when you brought the amp home so it is indeed an amplifier. http://forums.klipsch.com/forums/630990/ShowPost.aspx Like I'm hinting too above an amp can be dead silent by itself but if setup to be to sensitive almost any component attached is going to produce noise. So the problem can be blame on the sources and the amp to some extent. Craig Quote Link to comment Share on other sites More sharing options...
tpg Posted November 13, 2005 Author Share Posted November 13, 2005 Yes... sorry, the Forte Audio 1A is a 50 WPC pure Class A power amp desined by Nelson Pass. I am guessing it is really sensitive, but my CD-player also does put out a whole lot of voltage, I believe. I don't have any specs on this amp, other than it is 50 WPC Class A. [:S] Anyway, I rebuilt the digital pot last night a little tighter with a different input/output scheme and it seems to sound fine now (though I cannot get a good listen because there is a LOT of background noise from elsewhere right now). I couldn't hear any hiss/buzz/hum with my ear up to the speaker, but, as I said, it's noisy in here right now. So, I'll check it out a little later to be sure. I have read Nelson Pass' article about ground loops and took care not to run into this issue, which is why I was so perplexed by the problem. It seems to be working... so I am afraid to touch anything. [] EDIT: On a second close listen, I think I detect a small amount of buzz, but I don't think this will be audible at listening position (unless it is dead quiet in here, which is extremely rare). Quote Link to comment Share on other sites More sharing options...
leok Posted November 13, 2005 Share Posted November 13, 2005 The "hiss" you are refereing to may be hf oscillation .. this sometimes occurs with capacitively loaded op-amp or high performance feedback circuits. It also may be your grounding arrangement is feeding signal back from amp to source (this is sometimes refered to as a "ground loop").. Does the Amp have a grounded power plug? If so, try making sure nothing else in the system is using a grounded plug. You can also have a source (cd player, etc.) with a grounded plug, but then be sure the amp isn't using a grounded plug. The point is, if the signal is referenced to power ground at any point in the system, you want to have it not referenced to power ground anwhere else . otherwise you have a "loop." Is the power supply for your digital volume control grounded? Is that ground connected to signal ground? Signal ground should have one path between components and that's through the signal ground or reference circuit .. not through the power safety ground circuit. Leo Quote Link to comment Share on other sites More sharing options...
tpg Posted November 13, 2005 Author Share Posted November 13, 2005 I finally did a little testing after the setting became quieter and the right channel is very quiet. The left has a slight buzz that is only audible up close. So, I should have no problems at listening position. The CD-player does not have a grounded plug, and it is actually plugged into a totally separate power strip connected to a different wall outlet. The amp does use a grounded plug. The volume control doesn't plug into the wall. I am running it off of a battery supply. It requires +5V, so I am using a regulator and 9V battery (though I think I'll switch to two of my 4V SLA batteries in series sooner of later). I am currently keeping the analog signal grounds and the digital ground totally separate. I think this is the best way to run things, but am not totally sure. Funny thing about the volume control... I had to use around 80k series resistance between the input jacks and input of the digital pot before it would quit clipping when my CD-player is at full output. So, my 45k pot is actually now 130k or so, I guess. This is probably adversely affecting my amplifier in some way, but it sounds tons better than running it from my computer (even with a CD as a source in the computer instead of mp3's). I still have yet to get a serious listen in, though. Lots of homework and projects to do this weekend! Quote Link to comment Share on other sites More sharing options...
leok Posted November 13, 2005 Share Posted November 13, 2005 tpg, I'm glad things are getting better. How do you have the potentiometer(s) connected? I suggest the terminals at each end of the potentiometer resistance connect to source signal and signal-ground. The wiper and signal ground then connect to the amp signal and signal ground. (when the wiper is at its lowest point it should be at the signal ground end of the resistance. When the wiper is at full volume it should be at the source-signal end of the resistance.) If you are simply placing a variable resistance in series with the signal, it won't work very well and will be noisy. If you are connecting the wiper to the source, that would be noisey also and would explain the damage to the computer board .. the wiper goes to the amp. Leo Quote Link to comment Share on other sites More sharing options...
NOSValves Posted November 13, 2005 Share Posted November 13, 2005 Leo has you covered on the VC great advise. By the way Leo thanks for sending the Fisher referral my way I appreciate it. Craig Quote Link to comment Share on other sites More sharing options...
leok Posted November 13, 2005 Share Posted November 13, 2005 Craig, I hadn't heard what the final decision was with the Fisher .. so, that's good news all around. Leo Quote Link to comment Share on other sites More sharing options...
12ax7 Posted November 13, 2005 Share Posted November 13, 2005 The Forte 1A is a very nice Nelson Pass designed 50w ClassA SS amp. I used to sell these in the late 80's /early 90's. If I remember right it worked well with passive preamps. Leo, I'm sure Craig is the man to do this beauty justice. This piece is important to me for reasons far beyond hifi. It was the in the console of my hysterically funny Aunt Jean's home in New Haven where I spent a great deal of my childhood in the 60s. This amp powered many a serious party in that great house. It will always have a special significance for all those great memories it recalls. Richard Quote Link to comment Share on other sites More sharing options...
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