Sending Dared SL2000a in to repair hum

[audiocr381ve]

So I swapped my Polk Monitor 5's for a pair of Magnepan MG10's and can't hear a hum anymore.... Very strange.

The hum coming out of my Monitor 5's could be heard easily from my listening position. With the Maggie's swapped in, the hum is gone.

OP, have you tried swapping in a different pair of speakers?

[Stew]

I'd be willing to bet the Maggie's have a lower input sensitivity than the Polks. I think this is starting to make sense. From the research I've been doing tonight, a high amplifier input sensitivity (v) requires a higher volume setting to achieve the same sound level. A low speaker sensitivity (db/watt/meter) also requires a higher volume setting to achieve the same sound level. Either will reduce the noise floor.

I'm glad you found a solution but I'm not interested in different speakers. I need to look at the amplifier. My NAD has an input sensitivity of 1.1v. I've seen some amps as high as 1.75 v. Another option might be in-line attenuators (Rothwell, Goldenjack, etc.) but I'm concerned they might degrade sound quality. Any thoughts appreciated.

[heiney9]

My thoughts where the micros......
That said, if there is a major problem doing this........then you have other problems.
Comprende?

No. Flicking a running tube with your fingers or an object like a pencil is NOT RECOMMENDED. Poor advice is the major problem. To test for microphonics you can tap the base of the unit. If you don't get a ringing by doing that, then microphonics aren't an issue. Microphonics never manifest as a constant hum. You can create other problems by doing what you recommend to the tubes. Comprende?

H9

[heiney9]

I'd be willing to bet the Maggie's have a lower input sensitivity than the Polks. I think this is starting to make sense. From the research I've been doing tonight, a high amplifier input sensitivity (v) requires a higher volume setting to achieve the same sound level. A low speaker sensitivity (db/watt/meter) also requires a higher volume setting to achieve the same sound level. Either will reduce the noise floor.

I'm glad you found a solution but I'm not interested in different speakers. I need to look at the amplifier. My NAD has an input sensitivity of 1.1v. I've seen some amps as high as 1.75 v. Another option might be in-line attenuators (Rothwell, Goldenjack, etc.) but I'm concerned they might degrade sound quality. Any thoughts appreciated.

First off anything like an attenuator in the signal path is going to be detrimental, but you have to balance that with the fact that if it reduces the hum to acceptable levels or eliminates it, it might be a worthwhile compromise and may end up being less detrimental than listening a hum.

My Dared is dead quiet as far as humming, but there is a slight amount of hiss when I put my ear up to the tweeter, totally normal and acceptable.

H9

[heiney9]

Talk to Deltronics as I believe there is a way to actually reduce the gain of the pre-amp within the circuit. Probably swapping some resistor values. That would be much more effective than in-line attenuators. Have you tried using a lower Mu tube, like a 5965. It presents a little less gain.

H9

[nspindel]

As for the Auricaps, I replaced 10 coupling caps in my Dared MC-7P with Auricaps. Big improvement. I have a bit of a buzz in my Dared as well, have been lazy about it because it's not very noticeable. I do think it's because the gain is so high, I should look into some resistor swaps. They reall do have these things tuned to produce very high gain. If I go past 9:00 I'm at blast levels.

[Stew]

Thanks H9. I'll take your advice and call Deltronics on Monday. I haven't tried a lower Mu tube yet. I really like the sound of the Siemens triple mica's that I'm using and would hate to change. I may look into 5965's if I don't find another solution. If I can't lower the gain, what do you think about using an amp with a variable input (NAD C272 for example) as opposed to in-line attenuators? Would the variable inputs have any advantage over attenuators?

Same with mine nspindel. 9:00 is as loud as I dare.

Stew

[FTGV]

... what do you think about using an amp with a variable input (NAD C272 for example) as opposed to in-line attenuators? Would the variable inputs have any advantage over attenuators?

..... 9:00 is as loud as I dare.

Stew

But then attenuating the output of pre would then require a higher volume control setting to obtain the same level.

[Stew]

That's the idea. I have too much gain resulting in audible hiss and limited range on the volume knob (6:00 to 9:00). Lower gain or attenuators would increase the range of the volume knob hopefully to 11:00 or 12:00 and reduce hiss during quiet passages. I tried a makeshift attenuator (tube buffer turned off). It accomplished both but there was a loss of sound quality.

[FTGV]

OK so you have very limited usable range with your volume.For experimental purposes only you could try reconnecting the shorting links between the NAD's pre and amp sections.Then plug the Dared into one of the NAD's aux inputs and use the NAD's volume control as an attenuator to see if the noise is reduced.Ofcourse your Dared's volume would have greater useful range.

[FTGV]

I tried a makeshift attenuator (tube buffer turned off). It accomplished both but there was a loss of sound quality.

Powered off or do you mean a built in bypasss feature?

[Stew]

For experimental purposes only you could try reconnecting the shorting links between the NAD's pre and amp sections.Then plug the Dared into one of the NAD's aux inputs and use the NAD's volume control as an attenuator to see if the noise is reduced.Ofcourse your Dared's volume would have greater useful range.

Good idea! I didn't think of that. The volume knob on the NAD is labeled with db attenuation so it might help estimate how much I need to reduce the gain.

As for the tube buffer question, it was just powered off. It doesn't have a bypass feature per se but is designed to pass the signal (with some attenuation) when not powered on.

[audiocr381ve]

I'm glad we're on to something here. I wonder if I'm going to have the same problem with the LSi9's I'm picking up later this week.

Keep us posted Stew!

[heiney9]

Of the many Dared's owned on PF, Stew seems to be the exception. It could simply be a gear mis-match. It happens. I wouldn't worry too much Shane it's not really a speaker issue anyway unless one has ultra high sensitivity speakers, then all gear in the chain can be suspect. LSi 9's are not ultra efficient so no worries there.

H9

[zombie boy 2000]

And I would like to further clarify my experience with "Dared hum" by stressing just how slight it has always been. I'm talking "ear to the tweeter" slight. And yes... I've had it paired with inefficient speakers ranging from Lsi's to Totem Arros and rarely have been able to go above the 9:00 position on the volume dial without declaring war on the neighbors. Take all this info FWIW.

[erniejade]

I am sorry I am late on this. In the past not only for DJ use but, at home besides a panamax power conditioner, the other thing that has helped me with hum is http://www.swee****er.com/store/detail/HumX. HumX. For DJ use, I have 2 different rigs. When I am using my Bose L1 system with a LCD monitor, I get a terrible hum and putting a HumX on the LCD stops it. Granted I dont have that issue with my EV tops and KV2 bottoms, but, with the Bose i do.

When i had my system in the basement, I had one on my Turntable preamp. I had a low level hum that only would happen when the Tubebox II was plugged in. On the system up stairs, I never had a hum but in the basement, always and the HumX helped with that.

[sda2mike]

mine runs quiet...has some microphonics...like some others have reported, it's a loud mouth! it would be nice to have more of a sweep in the volume control...truth is, i'd probably set it back where the 2btl's are singing their best.

[george daniel]

mine is still quiet-- I think Brock hit it-- sometimes gear just does not mate/match well-- seen it before.

[Stew]

It could simply be a gear mis-match.
H9

At this point, I'm convinced my NAD amp doesn't like the high gain of the preamp. I'm using the amp portion of a NAD C370 integrated. I plan to upgrade to a seperate amp at some point but the NAD sounds great. I think I may have to fork out some dough to hear an improvement.

I emailed Deltronics about decreasing the gain and will keep everyone updated. Even when I upgrade amps, I assume the lower gain would be beneficial. Is there any reason not to decrease the gain?

[Stew]

H9 - Did you get my PM? Can you share the schematic for the Dared? I'd just like to learn. I won't pass it on to anyone without permission.
Thanks,
Stew

[Stew]

Thanks H9!

[Stew]

I heard back from Mike at Deltronics. He recommended against changing any resistor values to decrease gain and suggested I look at different tubes. He says 12au7, 12at7 and 12ax7 tubes will work. I'm running 12at7 (mu=60). Has anyone tried 12au7 (mu=20)? If so, how did they sound and how much difference did you notice in gain? H9 really liked 5965's (mu=47) as I recall. How much difference in gain did you notice with these?

I'd really like to leave the preamp/tubes alone and find an amp that plays well with it. For all of you with Dared's, what amp and speakers are you running and is hum a problem with your setup?

[sda2mike]

I heard back from Mike at Deltronics. He says 12au7, 12at7 and 12ax7 tubes will work.

this is great info! i was wondering about rolling in some other 12's...my dared is a bit of a loud mouth...a melodious loud mouth..i try some and post back...thanks!


mike

[heiney9]

I've used 5965, 6829, 6414, 7062 all inbetween a 12at7 and 12au7 and while the mu is lower, it doesn't make much difference for gain.

As far as using 12ax or au 7, when you start running gear out of it's optimal operating design it's usually a detriment to the sound. Certainly try it but i wouldn't hold my breath. The 12ax7 is high gain and draws more heater current.

[sda2mike]

i was thinking 12au7, to try anyway

[heiney9]

i was thinking 12au7, to try anyway

Try away, doubt it will make a significant difference.

[organ]

Hi Stew,
I also own a NAD amp (C272). Your problem sounds just like the one I had. The NAD is very picky with tube pre amps and the only easy way out was using the variable input on the amp. I've had some tube amps that were extremely quiet on it while others gave me serious hum.

DON'T swap the 12AX7 for a U7. It has lower gain but they're different enough electrically that you could end up damaging the pre.

*edit* just noticed the Dared uses 12AT7 (somebody correct me if I"m wrong). Still not a good idea to swap to a different type of tube unless the manufacturer says it's ok.

[Stew]

I'll be interested to find out if the 12au7's make a difference.

What amps work well with the Dared and SDA's? I love the NAD sound but it may be time to upgrade.

[organ]

Have you tried using the variable input on your NAD? It's there to prevent problems like this from occuring.
Honestly, when I was using the variable input, I did not notice any decrease in performance.

[Stew]

organ - Unfortunately my NAD doesn't have the variable input. I'm using the C370 which is essentially the C160 preamp and C270 amp all in one box. It has a variable output for the preamp but no variable input for the amp.

That's good to know that the C272 variable input didn't affect sound quality. With that in mind, I'm considering the NAD C270 or C272 or possibly a Parasound 1200 or 1500. Anyone have one of these you're interested in selling? Any other suggestions for an amp that likes Dared's and SDA's?