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 Post subject: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Tue 10, 2020 6:02 pm 
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Joined: Feb Fri 22, 2013 5:04 am
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Good afternoon,

I recently finished restoring and completely recapping (with 100% audio grade capacitors from Mouser) a really nice Heathkit AR-15 stereo receiver. It works perfectly on all functions — with the possible exception of the bass response. The receiver outputs evenly on both channels and the highs and midrange sound wonderful. However, the bass sounds “muddy” or not as crisp as I would have expected. It definitely is not as clearly defined as the bass from my Marantz 2225 and the Marantz is rated at 25 watts per channel versus the Heathkit’s 50 WPC. My speakers are capable of handling up to 80 watts continuous.

The four output transistors (40411) are original to the receiver. Is it possible that their age might contribute to the disappointing bass? Replacements are readily available from NTE at “reasonable” prices.

Perhaps the Heathkit will never live up to the Marantz and I’m just expecting too much.

I really look forward to y’all’s comments and suggestions.

Regards,

Jamie


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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Tue 10, 2020 10:41 pm 
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Location: Sunnyvale CA
Jamiecj5 wrote:
Good afternoon,

I recently finished restoring and completely recapping (with 100% audio grade capacitors from Mouser) a really nice Heathkit AR-15 stereo receiver. It works perfectly on all functions — with the possible exception of the bass response. The receiver outputs evenly on both channels and the highs and midrange sound wonderful. However, the bass sounds “muddy” or not as crisp as I would have expected. It definitely is not as clearly defined as the bass from my Marantz 2225 and the Marantz is rated at 25 watts per channel versus the Heathkit’s 50 WPC. My speakers are capable of handling up to 80 watts continuous.

The four output transistors (40411) are original to the receiver. Is it possible that their age might contribute to the disappointing bass? Replacements are readily available from NTE at “reasonable” prices.

Perhaps the Heathkit will never live up to the Marantz and I’m just expecting too much.


That's entirely possible, regardless of what anyone says, there are large easily-audible differences between different audio amplifiers, particularly in their bass response. . It's also possible there is something wrong with it (like a resistor in the the wrong spot) from the original kit construction.

Another possibility is that the "audio grade" capacitors - which is not a recognized industry standard, as far as I know - are microphonic, as many/most of them are, and the "mud" is the effect of microphonic feedback. A possible test for this is to move the amplifier *far away* from the speakers, like another room with long wires, and put it on a isolator of some sort, like a foam rubber pad. Don't move the speakers, that will change a lot of other things, too. Then see if it changes; if so, probably some microphonic parts.

Brett


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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Wed 11, 2020 12:09 am 
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Joined: Feb Fri 22, 2013 5:04 am
Posts: 195
During the restoration, I literally checked every single component against the assembly manual trying to make sure all resistors, capacitors and transistors were installed correctly and I verified the routing of every one of the wires in the multitude of wiring harnesses. That was a job, believe me. I did not find even one mistake. Also, interestingly, there are two separate red Heathkit service center stickers with two different dates a year apart stuck on the chassis. I wondered if the receiver went back to Heathkit due to assembly errors or some component failure.

I got the “audio grade” capacitors by using that sort option on Mouser’s website. It seemed fitting to use decent capacitors on a highly-regarded receiver of this vintage.

I will definitely try relocating the receiver distant from the speakers since I haven’t chosen a permanent location for it anyway.

Jamie


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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Wed 11, 2020 4:50 am 
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Location: Sunnyvale CA
Jamiecj5 wrote:
During the restoration, I literally checked every single component against the assembly manual trying to make sure all resistors, capacitors and transistors were installed correctly and I verified the routing of every one of the wires in the multitude of wiring harnesses. That was a job, believe me. I did not find even one mistake. Also, interestingly, there are two separate red Heathkit service center stickers with two different dates a year apart stuck on the chassis. I wondered if the receiver went back to Heathkit due to assembly errors or some component failure.


OK, good on that one, I guess. I have worked on endless parade of kit-built and "improved" audio equipment, the first step if it is a kit is to do exactly what you did. I would guess it's about 50/50 on a wrong component somewhere, which is pretty good if you think about it. And if you had met some of these guys.

Quote:
I got the “audio grade” capacitors by using that sort option on Mouser’s website. It seemed fitting to use decent capacitors on a highly-regarded receiver of this vintage.


Regular old Kemet or Panasonic industrial types are the way to go. As far as I know, there is no meaning to "audio grade" as a rating, but what it frequently means is gigantic loosely wound paper-in-oil or equally giant loosely-wound film caps with a cool-looking label. The best case is that it is a standard industrial capacitor from a major manufacturer that is embedded in a huge enclosure and a cool-looking label, because at least those function properly. In that case, they work fine and the only harm is the 5000% mark-up.

Brett


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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Wed 11, 2020 2:37 pm 
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Brett_Buck wrote:
...
Regular old Kemet or Panasonic industrial types are the way to go. As far as I know, there is no meaning to "audio grade" as a rating, ...

Brett

Hear, hear!

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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Wed 11, 2020 2:39 pm 
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Did you play/hear it before doing the re-cap? If not...it may just be the inherent design flaw if you don't find any other causes. BTW: How does it sound with headphones?


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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Wed 11, 2020 4:53 pm 
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Tbirdkid wrote:
Did you play/hear it before doing the re-cap? If not...it may just be the inherent design flaw if you don't find any other causes. BTW: How does it sound with headphones?
Listening through headphones is a useful test.

Also useful is to compare left vs. right channel. Most circuit faults will cause a substantial difference between the left and right channel.

One simple way to compare L vs. R:
    Get two 8 ohm load resistors of suitable wattage. Connect one load resistor to the left channel speaker output (+ and -). Connect the other load resistor to right channel speaker output (+ and -).
    Also connect one speaker from Left + speaker output to Right + speaker output. This speaker will respond only to the “difference signal” between left and right.
    Apply a mono audio signal to aux in (use a Y cord with 2 male RCA plugs)
    With the balance control centered there should be no sound coming out of the speaker.

-EB

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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Wed 11, 2020 5:03 pm 
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Also please be aware that the “Loudness” switch on the front panel generates a large amount of “bass boost” when switched on. To get “flat frequency response” the loudness switch must be in “off” position and the tone controls should be centered (12 o’clock position).

When this receiver was manufactured most of the available speakers didn’t produce much low bass response. Therefore many vintage stereo receivers dialed in more than +10dB of bass boost when their “Loudness” switch was in the “on” position. That much bass boost will sound muddy and boomy with today’s speakers.

-EB

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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Wed 11, 2020 6:47 pm 
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electricboyo wrote:
Also please be aware that the “Loudness” switch on the front panel generates a large amount of “bass boost” when switched on. To get “flat frequency response” the loudness switch must be in “off” position and the tone controls should be centered (12 o’clock position).

When this receiver was manufactured most of the available speakers didn’t produce much low bass response. Therefore many vintage stereo receivers dialed in more than +10dB of bass boost when their “Loudness” switch was in the “on” position. That much bass boost will sound muddy and boomy with today’s speakers.


The loudness switch was intended to compensate for the loss of high and low frequencies at low volume due to human hearing response. They may have also boosted it beyond the norm, but that was not the general standard at the time.

The other half of it is an interesting observation, depending on the speakers, some of them definitely *are* boosted. But not all, generally, speakers have gotten much more accurate with less coloring. Interestingly, that tends to give the impression of less bass volume, rather than more, because they aren't self-generating nearly as much of it. Headphones, on the other hand, tend to be *grossly* overblown at low frequencies . Not all, of course.

Brett


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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Wed 11, 2020 8:31 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Boston, MA USA
AR-15s were huge when I was in college and many of my friends, including my roommate, had them. He used it with big floppy acoustic-suspension speakers and the bass was tight-tight-tight, especially in comparison to the Sherwood S-5000II I had at the time.

-David


Last edited by dberman51 on Mar Thu 12, 2020 3:38 pm, edited 1 time in total.

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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Thu 12, 2020 1:23 am 
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I worked with the receiver this afternoon and evening and I tweaked a few things that most definitely helped. The receiver has separate, small potentiometers called Level Controls that are used for adjusting the left and right speaker volume for each of the sources, as Heathkit calls them. This keeps the amplifier volume relatively the same as you switch from AM, FM, Phono, Aux and Tape. I had the AUX pots cranked all the way up. I turned them counterclockwise about 1/8 of a rotation and that helped the Bass sound immensely.

Also, the Loudness switch MOST DEFINITELY has a significant impact on the Bass. I am still trying to decide if the receiver sounds better with the Loudness switch on or off.

The Treble control also has a feature where you pull it outward to instantly create a flat frequency response and the Bass and Treble controls become inoperative.

The bottom line is that I think the receiver is working just fine. I guess these ‘60’s vintage receivers simply require a little more user input to get them just right. I’m sure this is one of the reasons they are still loved and cherished so much.

I sincerely appreciate the insight and help from each of you.

Jamie


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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Thu 12, 2020 5:46 am 
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Jamiecj5 wrote:
I worked with the receiver this afternoon and evening and I tweaked a few things that most definitely helped. The receiver has separate, small potentiometers called Level Controls that are used for adjusting the left and right speaker volume for each of the sources, as Heathkit calls them. This keeps the amplifier volume relatively the same as you switch from AM, FM, Phono, Aux and Tape. I had the AUX pots cranked all the way up. I turned them counterclockwise about 1/8 of a rotation and that helped the Bass sound immensely.


I would interpret this as having the input levels set to the point that something is saturating the input. Line levels from sources are typically around 1.5V - 2V RMS at full volume of the source. If the input level exceeds the allowable range of the preamp front end, or if the first stage of your preamp (typically 20 db voltage gain) starts to clip, then it will introduce extreme distortion. It typically affects bass first, because for a given audio energy, the energy goes as the square of the frequency, meaning a given amount of high-frequency energy has less voltage amplitude than the same amount of energy at low frequencies.

Normally you would want to look for this sort of thing on a scope during testing, but it's not that touchy an adjustment. You lose a little gain if you turn down the input level more than necessary, but that is what a volume control is for.

Quote:
Also, the Loudness switch MOST DEFINITELY has a significant impact on the Bass. I am still trying to decide if the receiver sounds better with the Loudness switch on or off.


It's intended to have an effect, it's a fixed, switchable tone control, intended to make up fro the limitations of human hearing at low absolute amplitudes. It should act as if you turned up the bass and treble knobs on the tone controls. Of course, this is intentionally making the frequency response of the amplifier uneven, and like any tone controls, introduce resonant elements into the system that were not there before. I cannot tell you want you should like, but if you want to alter the frequency response to suit your taste, I would very strongly suggest using the tone controls, since you can dial them in to what you want. For my own money, while most of my preamps have tone controls (which is uncommon in high fidelity equipment), they remain switched out essentially all the time.

Brett


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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Thu 12, 2020 3:48 pm 
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The purpose of the level controls is to adjust the incoming audio levels so that normal listening level occurs when the volume control is turned up between 1/3 and 1/2 rotation. If the level is set too high, as yours was, this exaggerates the action of the Loundness switch, making the bass sound tubby. This is described in the owners manual if you can find one.

-David


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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Thu 12, 2020 6:32 pm 
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I built an AR-15 in my late teens. Like an idiot, I sold it decades ago. I just got one at auction that needs all the little green mylar replaced... they are all literally split open. I'll do a full restore while I'm in there.

I do not recall anything like mushy bass when I used mine. I had AR-2AX speakers to go with it. Also sold. :(

Do you have a signal generator? If so, or if you can get one and an oscilloscope, just feed it into the AUX input and trace the signal through all the way to the output....see where the bass is going astray. Look for phase issues between L and R channels as well. If it's a sweep generator, so much the better. You may want to buy or borrow some dummy loads so you don't blow out your ear drums. But maybe the issue is BEFORE the output amplifiers.

I don't think the output devices themselves could be responsible for this. They'd normally work, not work, or be noisy.

Good luck!! (to me, also lol)

EDIT: I just read the REST of the above posts. Look like you've got'er fixed. Congrats. :). You were probably distorting the preamp with the level set too high.

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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Thu 12, 2020 10:09 pm 
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Barry,

I did some research prior to starting the restoration and read a previous post, perhaps it was from you, regarding those mylar caps being cracked. Because of that post, I carefully checked all of the mylar caps in my receiver and found only four that looked suspect. I pulled and replaced them but each of them tested perfectly. I also pulled six more at random to test them and all were simply perfect. I sure hope I never have to go back in there and replace all of those mylars—-especially the ones on the control preamp circuit board. I will be really mad at myself for not replacing them when I had the chance.

I played the receiver for about an hour this afternoon and I am now very pleased with the sound. I used a little Bluetooth receiver plugged into the AUX jacks while playing music from Pandora via my iPhone. The Heathkit people in the 1960’s would have never dreamed of such a thing.

The comments from each of you have been incredibly educational. This is all proof that this is the best, most civil forum out there.

Regards,

Jamie


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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Fri 13, 2020 12:12 am 
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If this uses carbon composition resistors do test them as an out of tolerance resistor can affect the sound.


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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Fri 13, 2020 12:49 am 
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I'm sure the resistors are fine. These things are 50 years old, not 100. And Heathkit used pretty high quality parts during this phase of their existence. I have better things to do with life than test 500 resistors in-circuit. Actually I don't even recall if they are carbon or film. Rebuilding mine is a ways off yet.

If something doesn't work, that would be the time to start testing.

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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Fri 13, 2020 2:51 am 
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They could indeed be film.

I've seen mil spec carbon comp resistors be out of tolerance where I work, but there's no telling how many decades old they are.


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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Fri 13, 2020 3:07 am 
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My point was, out of spec does not bother me in the slightest, if the circuit works. :)

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 Post subject: Re: Heathkit AR-15 Receiver—“Muddy” Bass
PostPosted: Mar Fri 13, 2020 7:31 am 
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Hi Jamie, did you check out the feed back loop? that has a large effect on controlling the bass. I tried to look for the print but didn't see one.
Rich


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