Author Archives: Frederick Vobbe

About Frederick Vobbe

I have won numerous awards in broadcasting, electronic engineering, and civic service, and has been a finalist for the City of Lima Neighborhood Award numerous times, as well as nominated several times for the national Jefferson Award. I’m presently employed as Director of Engineering for Block Communications, Inc., and I’m Vice President and Chief Operator for the a group of television stations in Lima, Ohio. On the amateur radio side I’m the trustee for the KT8APR repeaters on 53.630, 145.370, and 443.625, and the W8AOH repeater on 146.745 operated by the Allen County Office of Homeland Security. These repeaters are analog, IRLP, and DSTAR.

I was the long time editor of the DX Audio Service, a non-profit radio magazine on audio cassette devoted to the medium wave DX hobby. The magazine ran from April 1985 till the National Radio Club shut it down in April 2015, I’m a member of B.P.O.E. #54, Eagles #370, and on the Salvation Army board of directors. I’m a past member of the Lima Host Lions Club where I served as President, VP, and on the board of directors. I hold the Melvin Jones Award, one of Lions Club top honors.

In my nearly 48 plus year radio career I’ve worked at recording studios, radio, and television stations. One of my other hobbies is collecting “oldies” from the years 1955 to 1990 that charted in the radio industry. I have my own automation system and Part 15 radio station where I can listen to “good” music. I also restore old recordings of radio jingles, airchecks, and production music from tape and records. Another hobby is designing circuits and home-brewing equipment such as audio processors, tube compressors, switcher & control systems, audio distribution amplifiers, and audio mixers.

I’m married to Debbie, who is amazingly tolerant of my audio/RF addiction. We have two children wonderful children, Jonathan and Constance, both grown up and have made their parents proud. I’m Originally from Toledo, Ohio, and lived in Lima since 1985. My ham radio call sign, W8HDU, was granted via the vanity callsign program, and was my grandfather’s call. He worked at Western Union during the days and at night he operated on 80 meters from his home in Toledo using a home brew transmitter and receiver, working the CW mode.

Review: Aukeys BM700 mic.

BM700 Microphone

Aukeys BM700 microphone, microphone clip, cord, and wind protection.

Recently I purchased an Aukeys BM700 microphone on the web site Aliexpress.

The microphone looks like a Neumann TLM102, but for $19.18(US) I was not expecting it to perform the same as a Neumann. The price of the microphone varies depending on the day with a high of $21.13 and a low of $18.10.

Another seller has the same microphone on sale except with a silver grill and a yellow foam filter. The thing which is confusing in some of the ads is the statement NO NEED phantom power mic when it’s sold as a Pro DJ and studio recording electret condenser microphone. I don’t think you can have it both ways.

The Aukeys BM700 looks good (physically) on the web site, so I figured if it was not a good sounding microphone I could take out the microphone element and replace it with something else.

I purchased three microphones as I wanted to provide a fair review of the product, averaging test results.

Testing was performed in my home studio using a Sony MZ-R70 digital mini-disc, bench testing with a Dayton audio analyzer, and Potomac AA-51/generator and AA-51A/analyzer with a preamp. The preamp was swept prior to the test for flatness, distortion, and noise.  Room calibrations were performed with the speaker and analyzer prior to measurement. Audio recordings were performed in the same location and setting. Sound files were recorded at 44.1 24 bit mono, and converted to 320kbps MP3 files for playing on the web.

The microphones were purchased on June 3rd, and arrived via China Post on June 11th, which is remarkable as the shipping was free! There was no trauma to shipping bag or contents. However one microphone did not appear to be in the original foam shipper. This microphone turned out to be dead.

Seller was contacted about the dead microphone on June 13th, and responded back with questions if I could determine why the microphone was dead. I responded the same day and told the seller since I had purchased three, and it was easy to determine the microphone was the dead and not cables or connectors. On June 16th the seller agreed to send a replacement. As of June 21st I’m still waiting for the replacement.

BM700 Mic

Description of product as seen on the Aliexpress web site.

The microphone pattern seems to be more of a narrow cardioid, and listening to the pickup on headphones as I turn the microphone left or right, up or down by at least 30-degrees, a drop off in level and frequency response is noted.

When the microphone is turned 180-degrees, (speaking into the back of the microphone), the level is louder that the sides, however the high frequency response is shelved from about 360 HZ upward.

When speaking into the microphone from eight inches away, without the foam wind protector, you will notice popping. Once the wind protector is on it limits the popping significantly.

The ad states, Overload-protection switch (-10 dB) minimizes distortion from loud sound sources. There is no such switch on the microphone.

The ad also states, Bass-reduction switch reduces room noise. There is no such switch on the microphone.

One would assume that a microphone with 3-pin XLR would be a balanced microphone. But the BM700 cord has a XLR at one end, and 1/8″ mini-phone plug at the other end.  There is no documentation on the wiring. Specifications claim 150-ohm.

3-pin XLR which comes on end of cord.

3-pin XLR which comes on end of cord furnished with BM700 microphone.

On the XLR end; pin 2 is the hot wire, while pins 1 and 3 are shorted together with the braid of the shield of the cable. On the 1/8″ connector side; ground is the shield, while tip & ring are the hot side, connected to pin 2 of the XLR. This seems to work well on my Sony minidisc recorder. However, since I could not find a way to take the microphone apart, verification of a balanced 150-ohm output from the microphone can not be verified.

BM700 Specification Sheet

BM700 Specification Sheet

The specifications state that the buyer receives a “user manual”,  In reality the manual is one sheet of glossy paper printed single side.

Specifications state frequency response of 20 Hz to 16 KHz, with a sensitivity of -38 dB, +/-2 dB.  The stated output impedance is 150-ohms, with a load of approximately 1,000 ohm.  In looking at the graph at the bottom of the sheet you can see the stated pattern of the microphone, as well as the claimed frequency response.   This response is somewhat misleading as I found depending on the azimuth and elevation of the microphone, with respect to the sound source, different values would peak and fall. For the most part I found the BM700 provided a boost in an octave centering around 5,300 Hz. You can hear the difference when compared to an Electrovoice RE20 microphone.

BM700 microphone grill

BM700 microphone grill

The microphones appears to be well made and machine tooling is clean. The gold grill of the microphone is especially bright and clean. The barrel is a non-reflective dull black. The male XLR is recessed into the bottom of the microphone with only 3/16 of an inch of silver connector showing. The pins for the XLR connector appear gold in color.

When looking into the grill I can see the microphone element which appears to be about one inch in diameter. There is no silk or cloth between the grill and the microphone element to prevent dust or foreign objects to come in contact with the microphone element or to prevent pops from speech. If I could take the microphone apart I would add this to the inside of the screen to prolong the life and usefulness of the microphone.

Examining the microphone body I can find no screws or indication how it’s put together. For the moment I’ll assume it’s either a tight threaded barrel or press fit.

BM700 Side View

Side view of microphone with microphone mount.

The microphone comes with a mic stand holder. The microphone is slid in, (somewhat tight), up to the grill ring, and is held in place by friction.

The tooling of the stand mount will accept both the U.S. standard microphone stand and European. Standard US mic stand thread is 5/8-27. The smaller European-style thread is 3/8-16. An adapter for the smaller European mount is furnished.

The microphone mount is not made out of metal, but it appears to be a hard ABS plastic with stretch strings to cushion the microphone. An adjustable wing-nut allows for positioning of the microphone in a range of 190-degrees up and down. I did notice some “slop” in the mount when the wing nut is loosened for elevation adjustment. It appears the hole for the screw is oval, rather than round. It does not impede the use of the mount, nor make it unusable, but it could have been machined better.

TESTING

Testing the microphone was very difficult. Depending on the axis of the mic in relationship to the test speaker, I would get different results.

The mic claims to have a response of 20 Hz to 16 KHz, but I found the low end lacking in many cases, and the high end rolled up 2 dB starting around 2,300 Hz.

Signal to noise ratio claims to be 78 dB, but my tests had it more in the range of 66 dB.

How does it sound?  Let’s compare an Electrovoice RE20 microphone, (broadcast standard), to the Aukeys BM700. When recording these files I did not make any changes to the sound level, or equalization.

  1. Speaking into the front of an Electrovoice RE20.
  2. Speaking into the front of a BM700.
  3. Speaking into the side of a Electrovoice RE20.
  4. Speaking into the side of a BM700.
  5. Speaking into the back of a BM700.
  6. Pop filter test of BM700.

CONCLUSIONS

This is by no means a “broadcast” or “professional” mic, nor would I use it for crucial sound recordings.

For the cost, $19.18(US), I would find it useful for my computer, recording speech on my mini-disc, or perhaps with an interface it could be a nice microphone for my amateur radio transceiver.

As you can see by the pictures the microphone is very good looking.

The price is justified for the body of the microphone. If I can find a way to take the mic apart, I think I’ll replace the microphone elements with my own professional ribbon or dynamic elements.

I’ve had a difficult time communicating with Aukey to get a replacement or refund on the microphone. This is not unusual when dealing with a seller who does not speak English as their primary or secondary language.  Based on the seller’s web page I also felt they could be a seller or general merchandise, and not someone familiar with electronic equipment, (such as professional microphones). Just be aware of this when you make your purchase.

Tape Machine R. C. – Part 1

This project came about when I tried to find a remote control panel for a Panasonic video tape recorder at work. I soon realized that other machines, even my own Otari MX5050B-II didn’t have a way to be operated from across the room. Yes, Otari made one, but I couldn’t find one that wasn’t beaten up or buggy. I decided to build my own remote control.

While in the design phase it occurred to me the control could be used on other tape machines. For this reason I decided to make the button PCB generic. Along with making it simpler from a design standpoint, it also allows consistency with the look of the panel. I don’t know about you, but having multiple remote control boxes and panels sitting around my room frankly makes the control room look trashy. The button panel furnishes LOW logic from the buttons, and LEDs light when the corresponding pin is grounded.

Since the logic of an Ampex, Tascam, Marantz, and Teac are all different, it’s easier to have a button control interface to the command logic and status. To obtain status I need to “hack” the machine for signal states to relay back to the buttons. For example; a Tascam A3300SX-2T does not have lighted remote control capability, but we can hack the machine so when the Play button is depressed, it lights up the “play” button on the panel.

The control interface uses dry contact relays to isolate the button presses from the machine. The relays are quieter than the ambient noises generated by the machine going into a modes.

BUTTON PANEL

Here is the button panel schematic.

Schematic for button assembly.

Schematic for button assembly.

The button PCB uses five Veetronix push button switches. These were chosen due to their feel, and proven longevity in a production studio environment. These are the same switches found in many broadcast video switchers. Four switches use .72″ square caps, and one switch (stop button) is a .72″ x 1.45″ rectangle cap. Each switch may be lit with an LED or incandescent bulb. There is the ability to have two auxiliary LEDs to indicate power and record enable. These are powered from the five volt buss and drawn to ground to light the LED.

PCB layout without ground plane.

PCB layout without ground plane.

PCB layout with ground plane. (blue is back, red is top)

PCB layout with ground plane. (blue is back, red is top)

The board measures 7.5″ x 1.6″ (19.05 x 4.05cm). A male DB15 is the interconnect device between the board and cable. The board is mounted to the back of a rack panel using 1.125″ hex standoffs. One set of standoffs connect to the frame of the DB15 to give it rigidity.

Below you see a 1RU rack panel which holds (2) remote control boards. The holes you see next to the buttons indicate remote ready, and record enable.

1RU panel for (2) control panels

1RU panel for (2) control panels

The “Play” button uses a green lens, the “Record” button uses a red lens. The rest of the lens caps are white. The lettering is produced by printing on an overhead projector (clear) sheet with a desk-jet, and cutting the plastic sheet so the word is centered, and dropped between the lens and the lens backstop.

PCB layout with Veetronix switches mounted on PCB

PCB layout with Veetronix switches mounted on PCB

When the board is assembled into the panel it looks like this.

Front of control panel. Left side is labeled, while right side is not.

Front of control panel. Left side is labeled, while right side is not.

Note: The power and record enable LEDs are not yet in this panel.

On the back side of the PCB is a male 15-pin DB connector. Using a 15 conductor cable with a female DB connector, this allows the panel to be installed, then plugged in, and attachment screws tightened to keep the cable from falling out.

Control Panel (back side) - Unfinished

Control Panel (back side) – Unfinished

As pointed out earlier, the control panel is generic, and contains (5) switches which contain LEDs to light the button, and (2) LEDs for “power” and “record enable”.

In part two of this article I’ll talk about interfacing machines, and how to deal with machines which have no provisions for mode status.

The Forgotten “H” Pad

As I work on my home studio I’ve run into level problems in the rack. My rack is not crooked, but my levels are! Anyone who has shot excessive level into an electronic device knows that bad things happen. Most of us don’t always stop to check levels before recording. You patch, hit record, and start recording, only to find out during playback that the recording is distorted.

Levels are measurements in decibels (dB) and be considered either as a power or as a voltage. The level in decibels is 10 times the logarithm of the ratio of two power levels. There are three ways to measure;

  • dBm: decibels or dB referenced to 1 milliwatt (.001 watt)
  • dBu or dBv: decibels or dB referenced to 0.775 volt (dBu is more commonly used)
  • dBV: decibels or dB referenced to 1 volt

I maintain a standard in my home studio of balanced 600-ohm audio at 0 dBu at all inputs/outputs on my patch bay. By doing this I don’t have to change the recording level on the machine based on the source device selected. Some older tube devices have zero reference levels of +4, +8, or +10 dB. I have even seen +12 dB outputs on some old 1940s equipment. The mission is clear. I need to have the level decreased to satisfy my house standard. The solution is the “H” Pad.

Hpad

An “H” pad is simply five resistors made in the shape of an “H” laying on its side with the values calculated to achieve a specific loss of signal while maintaining proper impedance. In the case of coming out of a tube compressor with an output of +8 dBu, and needing to have 0 dBu appear at the patch bay, the solution is to calculate for 8 dB of loss.

I hook up the equipment’s output to pins P1, P2, & P5. P1 is +, P2 is -, and P5 is ground.

From the pad, pins P3, P4, & P6, to the patch bay top row. Pin P3 is +, P4 is -, and P6 is ground.

Pad-Bay

What about ground? Without getting into a big debate, I’ve found the most practical solution in high RF/noise areas, and to prevent ground loops, is to float the ground at the output end and attach it at the input. Therefore, all inputs to my patch bay, (top row), are grounded.

From the chart below, and looking at the schematic of the pad above, we find the values of the resistors.

We need to decrease the level by 8 dB so R1, R2, R3, and R4 are 130-ohm, and R5 is 560-ohm. A caveat here; the assumption is that your output equipment is 600-ohm, and the input of the patch bay is 600-ohm. (Click on the chart to enlarge it and print).

H Pad Values

H Pad Values


The P.C. Board is very small and measures a whopping 1.50″ (38.1mm) x 11/32″ (8.731mm). It was made small for a reason. It will fit inside of equipment, or inside of a Switchcraft barrel connector. Most of the time I simply attach this to the output, securing the pad to the back of equipment with stand-off posts, and then run a wire to the patch bay.

Pad_PC3

The bottom is where you solder carefully. You need to use a soldering pencil with a very sharp point, thin solder, and solder at approximately 675-700 degrees.

The top of the board has the outline of the resistors. The proper resistors to use would be ones like the Xicon Metal Film Resistors which have a diameter of .07283″ (1.85mm), and a body length of .137″ (3.5mm). Available from Mouser Electronics you would want (4) 130-ohm resistors, and (1) 560-ohm resistors. These would be p/n: 270-130-RC, and 270-560-RC.

The 1/4 watt version of these resistors are tight on the board and one end needs to be raised up, but will work in a pinch. The 1/4 watt versions have a diameter of .0984″ (2.5mm) and length of .2677″ (6.8mm). They would be p/n: 271-130-RC, and 271-560-RC.

Both sides of pad.

Both sides of pad PC Board.


A small hole at each end provides a way to mount the board using a 2-56 screw and spacer, (Keystone p/n: 1797C), also available from Mouser.

Here is a completed 3 dB pad which is ready to be installed. Note the size of the PC Board in relation to the dime.

Complete 3 dB pad

Complete 3 dB pad


This pad will be installed inside the back panel of a tape machine, so the machine remains original spec, but the audio handed off to the patch pay and other equipment is at my required 0 dBu level.

Note: You don’t have to use a PC Board, although it looks better. But if you’re careful, keep your leads short, and use some electricians tape to prevent leads from shorting against the cabinet and other equipment, you can install a pad inside just about anything.