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 Post subject: RD7000 Audio Response
PostPosted: Feb Tue 05, 2019 3:31 pm 
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Joined: Aug Thu 23, 2018 1:11 pm
Posts: 27
After a few exchanges with board members in which the audio performance of the Royal 7000 was discussed, I decided to do a little investigating into the frequency response of the radio in its "stock" configuration and how the response could be improved. After all, this radio weighs almost as much as a 1960's stereo console, it should sound better than it does!

I observed both the acoustical response of the radio using a calibrated microphone placed close to the speaker to observe the "near-field" response, and the electrical response of the amplifier by observing the voltage appearing across the loudspeaker terminals. For all measurements the "tone" control was set to achieve the "widest" frequency response and the "volume" control was set to a point above the loudness tap on the pot.

Some findings:

(1) The high-frequency response of the radio is limited by that of the speaker itself. In the "stock" configuration, the acoustical response starts to roll off at about 5 kHz and the electrical bandwidth of the amplifier goes to about 8kHz, which is a reasonable match for the speaker. Reducing the feedback capacitor at the first audio stage from its current value of 2200pf to 220 pF, and reducing or eliminating the capacitor at the base of the pre-driver will increase the electrical bandwidth to almost 20 kHz, but the acoustical bandwidth increases to only about 8 kHz due to the speaker's behavior. This improvement is audible however, appearing as a little more "sparkle" in the highs.

(2) The lower -3db point of the "stock" amplifier's acoustical and electrical response is about 180 Hz. The electrical response is limited by the 0.33 uF capacitor at the base of the first audio transistor. Increasing this by a factor of 10, to 3.3 uF lowers the electrical "roll-on" frequency to less than 50 Hz, but the acoustical response corner only falls to about 110 Hz due to the speaker's response. This change is not really audible with the stock speaker, however...

(3) Connecting a mid-quality bookshelf speaker to the modified 7000 sounds REALLY good, confirming the assertion that the internal 3 x 5 inch speaker is the limiting factor for the radio once the electrical changes mentioned above are made.

Adding a small tweeter to the radio, with an appropriate crossover could address the limited high-frequency response while not affecting the external design of the radio. Improving the lower end response would most likely be impractical as a new speaker with lower resonant frequency but comparable size and mounting provisions would be needed.

John


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 Post subject: Re: RD7000 Audio Response
PostPosted: Feb Tue 05, 2019 3:52 pm 
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Joined: May Sat 25, 2013 11:15 pm
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Location: Central Iowa, USA
Nice research, John!


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 Post subject: Re: RD7000 Audio Response
PostPosted: Feb Tue 05, 2019 4:35 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5726
Location: Indianapolis, IN
Thanks for posting your interesting research, John !

Did you also try replacing the "failure-prone" tone control cap that Brett mentioned ?

Still trying to work-up the motivation to pull mine apart again...

John


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 Post subject: Re: RD7000 Audio Response
PostPosted: Feb Tue 05, 2019 5:06 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5726
Location: Indianapolis, IN
Hello again, John,

Just looked at factory Zenith schematic for my chassis and noticed that they'd already changed that feedback cap you mentioned to 220 pF. And the 4 X 6 speaker has 22 Ohm impedance to match the OTL audio output, so replacement in this model might be a bit more difficult...

John


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 Post subject: Re: RD7000 Audio Response
PostPosted: Feb Wed 06, 2019 7:10 am 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4292
Location: Sunnyvale CA
johnt46 wrote:
After a few exchanges with board members in which the audio performance of the Royal 7000 was discussed, I decided to do a little investigating into the frequency response of the radio in its "stock" configuration and how the response could be improved. After all, this radio weighs almost as much as a 1960's stereo console, it should sound better than it does!

I observed both the acoustical response of the radio using a calibrated microphone placed close to the speaker to observe the "near-field" response, and the electrical response of the amplifier by observing the voltage appearing across the loudspeaker terminals. For all measurements the "tone" control was set to achieve the "widest" frequency response and the "volume" control was set to a point above the loudness tap on the pot.

Some findings:

(1) The high-frequency response of the radio is limited by that of the speaker itself. In the "stock" configuration, the acoustical response starts to roll off at about 5 kHz and the electrical bandwidth of the amplifier goes to about 8kHz, which is a reasonable match for the speaker. Reducing the feedback capacitor at the first audio stage from its current value of 2200pf to 220 pF, and reducing or eliminating the capacitor at the base of the pre-driver will increase the electrical bandwidth to almost 20 kHz, but the acoustical bandwidth increases to only about 8 kHz due to the speaker's behavior. This improvement is audible however, appearing as a little more "sparkle" in the highs.

(2) The lower -3db point of the "stock" amplifier's acoustical and electrical response is about 180 Hz. The electrical response is limited by the 0.33 uF capacitor at the base of the first audio transistor. Increasing this by a factor of 10, to 3.3 uF lowers the electrical "roll-on" frequency to less than 50 Hz, but the acoustical response corner only falls to about 110 Hz due to the speaker's response. This change is not really audible with the stock speaker, however...

(3) Connecting a mid-quality bookshelf speaker to the modified 7000 sounds REALLY good, confirming the assertion that the internal 3 x 5 inch speaker is the limiting factor for the radio once the electrical changes mentioned above are made.

Adding a small tweeter to the radio, with an appropriate crossover could address the limited high-frequency response while not affecting the external design of the radio. Improving the lower end response would most likely be impractical as a new speaker with lower resonant frequency but comparable size and mounting provisions would be needed.

John



That's generally consistent with my testing. I am sure that the rolloff at 180 Hz was intentional, to keep otherwise unplayable bass out of the speaker (which will just distort everything when fed low-frequency signals). The same thing is true of many radios.

Point 3 is true in droves, the speaker and enclosure is essentially the only limiting factor in the audio performance. It's absolutely amazing (mono) when played through a competent audio system even straight out of the earphone jack. The same is true of the 3000 and 2000. The 2000 can just about drive an external hi-fi speaker by itself (connecting the internal speaker leads directly to an external speaker) and that works very well as long as you don't expect too much volume out of it. But they work perfectly with connections to a preamp, or an externally-powered amplfier. At work I use my old beaten-up Royal 3000 and $30 "50 watt" Class D computer speakers and it's as good as many starter off-shore mid-fi systems - but VASTLY cheaper.

Brett


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 Post subject: Re: RD7000 Audio Response
PostPosted: Feb Wed 06, 2019 12:35 pm 
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Joined: Aug Thu 23, 2018 1:11 pm
Posts: 27
John - I DID replace the 1uF electrolytic at the wiper of the volume control, more out of a sense of caution than anything else. The actual value of this cap does not greatly affect the frequency response, but if the cap were to become leaky that current would flow through the volume control and appear as a noisy control.

Brett - Good point about driving the speaker below the its resonant frequency. I should probably revisit the value of the input coupling capacitor at the first audio stage which forms dominant pole of the amplifier. While I don't hear any "complaints" from the speaker, the electrical bandwidth certainly does not have to extend down to ~50Hz which is now the case.

I did "lash up" a small (1-1/2 inch diameter speaker from my junk box and a 1 uF series capacitor "crossover" to see whether the addition of a tweeter would make an audible improvement in the radio. The measured near-field acoustical response WAS broader, extending to a little over 10 kHz; and I think I heard a difference, music on FM SEEMED a little "brighter". This could just be wishful thinking, I'll have to live with it for a while.

John


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 Post subject: Re: RD7000 Audio Response
PostPosted: Feb Thu 07, 2019 5:06 am 
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Joined: Nov Sun 08, 2009 9:48 pm
Posts: 478
Location: Fredericton, Canada
Interesting research John. If I could find a suitable new speaker with the right impedance I would make the changes you outlined just to see how it would improve the radios sound. I've already done the electrolytics on one of my R7000's and will be going back in to do the cap replacement for tone that Brett mentioned.

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John Jarratt


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 Post subject: Re: RD7000 Audio Response
PostPosted: Feb Thu 07, 2019 4:00 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5726
Location: Indianapolis, IN
Hello Copiertech,

Please let us know the results of replacing that tone cap !

Thanks !
John


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 Post subject: Re: RD7000 Audio Response
PostPosted: Feb Thu 07, 2019 4:06 pm 
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Joined: Nov Sun 08, 2009 9:48 pm
Posts: 478
Location: Fredericton, Canada
Will do.

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John Jarratt


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