Signal Amplifiers (Amps, Preamps, Distro Amps) - Page 93 - Canadian TV, Computing and Home Theatre Forums

 
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post #1381 of 4157 (permalink) Old 2009-09-30, 12:58 AM
 
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Mixed Results on cheapie Radio Shack distribution amps

I just did an installation for my dad last week.
DB2 on chimney...>60ft RG6 straight into an LG plasma.

Even with 60ft RG6, all Toronto channels banged in almost full strength as the chimney had LOS to CN Tower. However, we only got 7.1, 23.1 and 29.1 probably because of a tree blocking LOS to GI.

I finally cut the RG6 at the grounding block, so it's now ~30ft. Tried the said 2-way distribution amps (also tried a +10db booster) and we lost 7.1 !! Took them cheapie amps off and 7.1 came back even with the cut RG6 and we had to use connectors and stuff.

I put 'mixed result' in title because those amps helped a little bit at my house back then. However, the CM7778 is what makes the difference. I had fed the down lead of the 7778 into these cheapie amps and got good results as I fed 9 devices LOL.

It certainly looks to me that cheapie amps kills borderline signal and that marks their official retirement.

LG55LW5000/DenonAVR791/MirageOMD5+Nanosat+MM8/ClearStream2/CM7778+3414
OptomaHD33/YamahaRXV1075/KEF3005/Velo15"
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post #1382 of 4157 (permalink) Old 2009-09-30, 09:03 AM
 
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CM Preamps - Inputs: Combined and Separate

Regarding my previous inquiry, Thanks all for your thoughtful replies:

Quote:
roger1818 - My guess (and it is only a guess) is that the switch would:...
Your guess is very close to what effectively seems to be occurring, at least in the CM-7777, although it appears that no amplifiers are actually switched off.

Quote:
Jase88 - "Combined or separate" pre-amps have two antenna inputs: VHF, and UHF or combined.
Jase88, your description is is pretty much operationally correct, although doesn't address what is actually happening internally.

Quote:
balm - Mine doesn't say this
Well, not exactly sure what mine said either, since the instructions are stored away elsewhere (a few hundred miles from my current location). But, the CM-0264DSB does have both a tunable notch for FM and a FM band-reject filter, whereas the CM-7777 has only the FM band-reject filter. And there were some special instructions regarding the activation of FM rejection circuitry (in addition to tuning the frequency of the notch to match the station to be rejected) for the CM-0264 that was not present in the instructions for the CM-7777.

Ken Nist (KQ6QV) kindly provided a partial schematic of the "Combined/Separate" switching arrangement for the CM-7777:



Ken's partial schematic well supports his, and others, premise that the VHF and UHF signals are strictly processed separately. And for the CM-7777, it also somewhat alleviates my concern about excess thermal noise from the UHF portion being injected into the VHF section. Most likely the filters eliminate such.

Thanks for the help.

======================================

Another topic:


A while back intravino asked about the Winegard AP-3700 preamp:

Quote:
intravino - Does someone have or had the Winegard VHF/FM AP-3700 Preamp?
Intravino, the AP-3700 (VHF-only preamp) was compared directly against the CM-7777 (using only VHF section, "Separate", UHF input loaded with a 75 Ohm terminator*) for channel 13 reception.

* The 75 Ohm terminator seemed like a good thing to do at the time, because of pre-amplifier stability concerns and, of course, for weather protection. But, still I'm unsure whether it affects noise levels into the combined output or not. This probably depends on how good the internal VHF/UHF filtering is. The thermal noise (kTB) originating and amplified from a resistor source (terminated load on input) is almost certainly greater than noise originating from purely reactive loads (unterminated inputs).

The driving factor for the preamp comparison was a concern that a strong FM station at 89.7 MHz would not be rejected sufficiently by the CM-7777 preamp (without the additional notch that the AP-3700 had). Note: 89.7 MHz is very close to the upper edge of channel 6 at 88 MHz, so it is difficult to suppress 89.7 MHz without also affecting channel 6. Guess suppression of FM signals at the lower end of the FM band vs gain on TV channel 6, or maybe even channel 5, is a tradeoff that is made by the designers of the FM band-reject filters that are used in various preamps.

Before installation, a system noise figure analysis was performed. The two preamps provided very close to the same system noise figures. If the CM-7000 (DTV converter used in the comparison test) was assumed to have a greater than ~8 DB noise figure, then the CM-7777 had a small advantage. If the CM-7000 had a noise figure less than about 5 dB, then the AP-3700 gained the advantage.


Specs for the two preamps:
AP-3700: Gain=17 dB; Noise Figure= 2.6 dB

CM-7777(VHF): Gain= 23dB; Noise Figure= 2.8 dB
Test conditions were:
- Pair of vertically stacked Winegard YA-1713 Yagi at 62 feet above ground level

- Both preamps located between the stacked antennas (about 4 feet RG-6 connecting each antenna through a combiner, Winegard CC 7870)

- Transmission line 85 feet (RG-6)

- Receiver front end, Channel Master CM-7000
- Strongest undesired signals:
* FM station at 89.7 MHz (~ -24.5 dBm, ref. FM Fool), and a
* UHF station (~-37.8 dBm, ref. TV Fool)
- Desired signal: channel 13 at -101.4 dBm (ref. TV Fool)

- Both preamps were tested for about five to ten minutes each, then the tests were repeated.

The indications used for comparison:
CM-7000 signal goodness indicator (probably a bit error rate (BER) indication), plus picture quality. The picture quality remained visually near perfect even with low numbers on the CM-7000 indication.
The stacked antennas were rotated a fixed distance in azimuth for each test, to ensure less than 100% indications. When the antenna was pointed at bore sight (dead-on), the signal 'goodness' indicated on CM-7000 was close to 100%, sometimes slightly less.

The monitoring was accomplished with the help of two assistants, one at the TV watching and announcing CM-7000 indications, another relaying CM-7000 readings (approximately every second or when the numbers changed) to person at top of tower.

A Channel Master signal level meter (similar to Sadelco FSM-3 or FSM-4) was available for other measurements (such as optimizing stacking distance or spacing, and determining bore sight/center beamwidth direction). The signal level meter was not used to directly compare preamps. The signal level meter doesn't directly help in comparing signal/noise ratios (S/N).

Tests for signal overload were performed by inserting and removing a Pico Macom HLSJ (VHF hi-low splitter/combiner). The HLSJ, in combination with a 75 Ohm terminator on the unused port, can be used as an excellent FM reject filter.

Not being sure about the CM-7000 response to FM signals, there was also a concern about overload of the CM-7000 from the FM signal or any other signal that may have been be simultaneously presented to the CM-7000 input. Tests for CM-7000 overload were performed by insertion of attenuation at the CM-7000 input and looking for changes in signal "goodness' on channel 13. No indications of CM-7000 overload were observed.

If there had been any indication of overload, it would have probably occurred with the CM-7777. During setup it was confirmed that FM rejection of the the AP-3700 (at the notch frequency) was much superior to the CM-7777. Plus the lower gain of the AP-3700 may have helped with system dynamic range.

There was virtually no difference in the performance of the two preamps.

In the end, for no particular reason, the CM-7777 was selected.

Last edited by tripelo; 2009-09-30 at 01:15 PM. Reason: For clarification, hopefully :)
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post #1383 of 4157 (permalink) Old 2009-09-30, 10:51 AM
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Quote:
And there were some special instructions regarding the activation of FM rejection circuitry
the 0264DSb says this only:

The trap is tuned to 125 Mhz with trimmer fully open and to 88 Mhz with trimmer fully meshed

Quote:
Broadband trap with switch in the IN position, eliminating FM signal. If FM signal are desried, slide the switch to the OUT position.
FM interference indicated by herringbone pattern on VHF channel (2 thru 13). If this interference is present, put FM trap switch in the IN position. IF interference is still present , turn tunable FM trap slowly while observing channel with strongest interference.

The tunable trap is continuous tunable and shipped from factory tuned out of FM band - about 125 MHz.
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post #1384 of 4157 (permalink) Old 2009-09-30, 11:27 AM
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Quote:
Originally Posted by tripelo View Post
Ken Nist (KQ6QV) kindly provided a partial schematic of the "Combined/Separate" switching arrangement for the CM-7777:
Wow! I really like the way they designed this. They have a band splitter on the UHF/combined input with the UHF portion going to the UHF amp. This ensures that in either mode, strong VHF or FM signals won't overload the amp and create harmonic distortions on UHF (if the VHF amp overloads, the band combiner would filter out harmonic distortion on UHF).

The only thing I noticed is that while they do terminate the terminate the VHF portion of the combined input with a 75 ohm resistor when the switch is set to separate, the VHF input is left floating when the switch is set to combined. Probably not a big deal since there aren't any active components attached and the trace is probably pretty short, but I would have had it terminated just to be safe (it would only have required three traces to be moved and one new trace assuming they have two or more layers on the PCB). This modification would also then use all terminals on the switch, so that you don't end up with a short antenna inside the amp (which also probably isn't a big deal since it is so short).

Knowing this, to be safe I would put a terminator on the VHF input if you are using it in combined mode, which is probably a good idea anyway for weather protection.
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post #1385 of 4157 (permalink) Old 2009-09-30, 12:00 PM
 
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Quote:
roger1818 -They have a band splitter on the UHF/combined input with the UHF portion going to the UHF amp.
Yes this band splitting definitely helps with dynamic range and reduces IMD products. Blonder Tongue (BT) has for years effectively used band splitting. On all the BT preamps that I am familiar with, they also split the VHF band with separate amplification for VHF low-band and VHF high-band. This gave BT a significant advantage in strong signal areas.

Quote:
roger1818 - to be safe I would put a terminator on the VHF input if you are using it in combined mode, which is probably a good idea anyway for weather protection.
Your comment prompted the addition (edit) a comment into the previous post:

Quote:
* The 75 Ohm terminator seemed like a good thing to do at the time, because of pre-amplifier stability concerns and, of course, for weather protection. But, still I'm unsure whether it affects noise levels into the combined output or not. This probably depends on how good the internal VHF/UHF filtering is. The thermal noise (kTB) originating and amplified from a resistor source (terminated load on input) is almost certainly greater than noise originating from purely reactive loads (unterminated inputs).
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post #1386 of 4157 (permalink) Old 2009-09-30, 12:23 PM
 
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Thanks Tripelo For your answer.

I was looking into the AP-3700 for two things:

Improving my CH 13 VHF reception if possible compared to my CM 7777, so I see it would be a tie with the CM 7777.

Second: I was looking for a FM amplifier for my Winegard HD-6000 and SONY HD tuner XDR-F1HD.


Thanks,

Intravino
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post #1387 of 4157 (permalink) Old 2009-10-02, 10:22 AM
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Kitztech Amp

What's the deal with the Kitztech and powering down?

What is the running time limit of the amp before you have to shut it down (10 seconds), allow it to reset and fire it up again?

Should the Kitztech be shut off if you are not using it/have the TV off?

This seems like a major limitation, unpluging and pluging in the amp every time the TV goes on/off...

91XG (47'), YA-1713 (42'), CPA-19, DB4e (37'); Denon AVR-1312
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post #1388 of 4157 (permalink) Old 2009-10-02, 10:47 AM
 
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My Kitztech was sent back for repair because it shutdown permanently. It did amplify anymore. I had a timer on the amp, it was sent from 5:00 pm to 1:00 am.

I going to see what he will say.
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post #1389 of 4157 (permalink) Old 2009-10-02, 11:31 AM
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Smile Provo pre-75 preamp

this is my first post and i am a complete newbie

i just bought the above preamp and spec sheet reads
vhf gain 15 db
uhf gain 16 db
response flatness +-2db
noise vhf 5db uhf 8db
have not installed yet so don't have to climb tower again

what are thoughts on these specs ??
this is a response to post on page 89 on this thread
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post #1390 of 4157 (permalink) Old 2009-10-02, 12:20 PM
 
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lowrider...

I cannot claim to be an expert on this but the noise figure concerns me - 5db on vhf and 8db on uhf is high when you compare it with Channel Master and Winegard amps that are in the 2db to 3db range. That much noise will probably bury a marginal signal.
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post #1391 of 4157 (permalink) Old 2009-10-02, 01:52 PM
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Thumbs down re: Provo pre-75 preamp

Quote:
Originally Posted by lowrider View Post
this is my first post and i am a complete newbie

i just bought the above [Provo pre-75] preamp and spec sheet reads .. noise vhf 3.5db uhf 8.5db. what are thoughts on these specs ??
My response, is that it is extremely unlikely to help with reception, and very likely to make reception worse.

Return it for a refund, if you can.

Cheers
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post #1392 of 4157 (permalink) Old 2009-10-02, 01:55 PM Thread Starter
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Thumbs down The Provo Pre-75 is garbage

lowrider, posts #1330 through 1334 and some other posts already say that the Provo-75 is garbage. Now it is confirmed further.

Having said that, If by some chance a tester can confirm analytically that their quoted noise figure is missing a leading decimal point then it would be an excellent preamp.
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post #1393 of 4157 (permalink) Old 2009-10-02, 04:35 PM
 
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Question Winegard AP-8700 gain curve requested

Has anyone perhaps measured the gain curve of this amp? I requested it from Winegard but their response was that they don't have it The reason is that I suspect the gain value above channel 51 since my channels 60, 65 and 66 are all noticeably weaker after this preamp and +-100ft of RG6 plus splitters than they are at the input of the pre-amp - telling me that the gain for these channels are less than the about 10db of losses (line and splitters) after the pre amp. The 950Q Hauppauge Signal Monitor reading drops from about 25 to 21 on channel 66 (OMNI2 in Ottawa) Channels 43 and below are good. The specs states that it is an 82 channel pre-amp with a gain of 19dB in UHF
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post #1394 of 4157 (permalink) Old 2009-10-02, 06:27 PM
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Kitztech

@goforit: An amp shouldn't need to be shutdown whatsoever. My CM7777 has been operational for months, and I've never had to power it down.

If an amp is shutting down, and requires a power cycle to revive it--I wouldn't recommend buying it.
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post #1395 of 4157 (permalink) Old 2009-10-02, 07:04 PM
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Same here Jase. Ive had CM preamps for years, never had a problem, and never once did I ever have to power one down.

They barely get warm to the touch and IMHO they are still the best around.

Cheers

Bob
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